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Analog Integrated Circuits and Systems(EE002-3.5-3)

Assignment

DC-COUPLED, SELECTABLE-GAIN HEADPHONE AMPLIFIER CIRCUIT

Table of Contents:

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Op Amps 3

Coupling 10

Selectable Gain 12

Construction of Circuit 18

Calculations of Circuit 20

Design and Simulation of Circuit 22

Benefits of Circuit 26

Practical 26

Conclusions 29

References 29

Op - Amps:

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Operational amplifiers (op amps) were originally used for mathematical operations in analog

computers. They typically have 2 inputs, a positive (non-inverting) input and a negative

(inverting) input. A signal fed into the positive (non-inverting) input will produce an output

signal which is in phase with the input. If the signal is fed into the negative (inverting) input, the

output will be 180 degrees out of phase when compared to the input.

Op - Amps Operation:

Below is a simple buffer circuit. The input impedance of an op amp is extremely high (on the

order of 1012 ohms). It might be used if the input signal to the op amp was coming from a source

which could supply almost no current. The output of the op amp can easily drive 1000 ohms or

more. The output, when used as a buffer, will theoretically be identical to the input signal. Can't

say it is as identical because there is a small amount of distortion in all amplifier circuits. The

distortion in this circuit would be extremely low and would most likely be inaudible.

Op - Amps as Amplifiers:

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An op amp can also easily amplify a signal such as audio. The diagram below shows the circuit

for an op amp that would give an output signal twice as large as the input. Op amps don't like

errors. To get amplification, you induce an error in the signal going back to the negative input of

the op amp. An op amp will do everything in its power to get the signal on the negative input to

match the signal on its positive input. To get an output that's twice as large as the input, you use

2 equal value resistors as a voltage divider to reduce the return (feedback) signal at the negative

input by half. If the return signal doesn't match the input signal, the op amp will increase the

output until the signal returned to the negative input is the same as the input to the positive input.

Since the voltage divider cuts the signal in half, the signal at the output must be doubled. We can

create any amount of gain needed by changing the value of one of the resistors in the feedback

path. The actual limit of gain will be determined by the op amp design. When using an op amp as

a non-inverting amplifier, the gain will always be greater than or equal to 1. To get a gain of less

than 1, we need to use a voltage divider on the input signal.

Op - Amps Inverters:

An op amp can produce a signal which is 180 degrees out of phase (inverted) with respect to the

input signal. The diagram below shows an op amp used as an inverter. To use an op amp as an

inverting amplifier, you must send the signal into the negative input instead of the positive input.

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The op amp will do everything it possibly can to make the voltage (signal) on the negative input

match the positive input. You can see that the positive input is connected to ground. It's shown as

being connected through a resistor but the resistance to ground in unimportant. What is important

is that the positive input has no signal (it's connected to the reference, ground). This means that

the op amp's negative input will have no visible (voltage) signal on it. When we are driving the

negative input it will act as a virtual ground. The input is converted from a voltage drive to a

current drive. The change in current is what drives the op amp. This is important to know

because when we look at the negative input with an oscilloscope, we will see no signal (when the

circuit is an inverting amplifier). The op amp inputs had very high impedance. While this is true,

when using the inverting input with feedback (which is necessary for audio reproduction), the

input impedance becomes the value of the input resistor.

Op - Amps Error Correction:

An op amp is commonly used in a circuit where error correction is required. Op amps can't

(generally) supply a large amount of current at its output. If a signal is fed to the positive input of

an op amp and the op amp is driving a circuit which can supply a large amount of current (like

the regulator), the output of the whole system can be fed back into the negative input of the op

amp. This will allow the op amp to compare the output (of the whole system) to the input signal

and correct as needed. If the op amp is used in a circuit which needs little current at its output,

the op amp can still monitor the output and correct as needed.

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The diagram below has a resistor in series with the output of the op amp and the load which is to

be driven by the op amp. The resistor represents anything that may be between the op amp and

the load. The resistor could actually be a long run of wire, resistance in the copper of a printed

circuit board or anything else that may cause the signal to be distorted. If the op amp didn't

monitor the signal at the load, the signal would be distorted (in this case, the simple series

resistance would only reduce the signal level). If the resistor would instead be an external circuit

designed to increase the output current (such as the transistors, resistors, capacitors of a power

amplifier), the output of the op amp may not even resemble the final output signal. The op amp

would do everything possible to get the final output to match the input signal.

LF356N:

This IC would act as our Operational Amplifier. They (family) are the first monolithic JFET

input operational amplifiers to incorporate well matched, high voltage JFETs on the same chip

with standard bipolar transistors (BI-FET Technology). These amplifiers feature low input bias

and offset currents/low offset voltage and offset voltage drift, coupled with offset adjust which

does not degrade drift or common-mode rejection. The devices are also designed for high slew

rate, wide bandwidth, extremely fast settling time, low voltage and current noise and a low 1/f

noise corner.

Advantages

Replace expensive hybrid and module FET op amps.

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Rugged JFETs allow blow-out free handling compared with MOSFET input devices.

Excellent for low noise applications using either high or low source impedance - very

low 1/f corner.

Offset adjust does not degrade drift or common-mode rejection as in most monolithic

amplifiers.

New output stage allows use of large capacitive loads (5,000 pF) without stability

problems.

Internal compensation and large differential input voltage capability.

Applications

Precision high speed integrators

Fast D/A and A/D converters

High impedance buffers

Wideband, low noise, low drift amplifiers

Logarithmic amplifiers

Photocell amplifiers

Sample and Hold circuits

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Offset Voltage (Defects):

In an operational amplifier, the input offset voltage is the difference in the voltage between the

+ and - input pins when negative feedback is applied. Although ideally 0, in an operational

amplifier, there is a difference between VBE of the + and - input transistors in the input-stage

differential amplifier, which becomes the input offset voltage.

In actual use; for example when using an operational amplifier with an input offset voltage of

V10 ≤ ±5mV, if ×100 amplification is created, there will be an error of up to ±5×100= ±500mV

at the output. Therefore, when using a circuit that handles very small input signals, you must

either select a product with a small input offset voltage, or adjust the offset externally.

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Measurement method: Assuming VIN = 0 in the above circuit, the input offset voltage can be

measured by dividing the output by 100. The measurement circuit in this case would be as

follows.

Input Offset Current (Defects):

There is a difference in the input current that flows in or out of each of the input pins, even if the

output voltage of the operational amplifier is 0 V, due to the fact the pair characteristics (hFE,VBE)

of the differential transistor do not match. This difference is known as the input offset current

IIO=|IB+-IB

-|

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The drop in voltage that occurs due to this different current flowing through the signal source

resistors (RS) connected to the operational amplifier inputs causes the offset voltage to rise.

Note that the input offset current causes an error in the output value in the same way as the input

offset voltage

Coupling:

Amplifier Coupling:

Almost every electronic device contains at least one stage of amplification. Many devices

contain several stages of amplification and therefore several amplifiers. Stages of amplification

are added when a single stage will not provide the required amount of amplification. For

example, if a single stage of amplification will provide a maximum gain of 100 and the desired

gain from the device is 1000, two stages of amplification will be required. The two stages might

have gains of 10 and 100, 20 and 50, or 25 and 40. (The overall gain is the product of the

individual stages-10 X 100 = 20 X 50 = 25 X 40 = 1000.)

Figure below shows the effect of adding stages of amplification. As stages of amplification are

added, the signal increases and the final output (from the speaker) are increased.

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Whether an amplifier is one of a series in a device or a single stage connected between two other

devices, there must be some way for the signal to enter and leave the amplifier. The process of

transferring energy between circuits is known as Coupling. There are various ways of coupling

signals into and out of amplifier circuits.

Direct Coupling:

The method of coupling that uses the least number of circuit elements and that is, perhaps, the

easiest to understand is direct coupling. In direct coupling the output of one stage is connected

directly to the input of the following stage. Figure below shows two direct-coupled transistor

amplifiers.

Notice that the output (collector) of Q1 is connected directly to the input (base) of Q2. The

network of R4, R5, and R6 is a voltage divider used to provide the bias and operating voltages

for Q1 and Q2. The entire circuit provides two stages of amplification.

Direct coupling provides a good Frequency response since no frequency-sensitive components

(inductors and capacitors) are used. The Frequency response of a circuit using direct coupling is

affected only by the amplifying device itself. Direct coupling has several disadvantages,

however. The major problem is the power supply requirements for direct-coupled amplifiers.

Each succeeding stage requires a higher voltage. The load and voltage divider resistors use a

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large amount of power and the biasing can become very complicated. In addition, it is difficult to

match the impedance from stage to stage with direct coupling. The direct-coupled amplifier is

not very efficient and the losses increase as the number of stages increase. Because of the

disadvantages, direct coupling is not used very often.

Selectable Gain:

To select gain for a dc - coupled circuits we can either use potentiometer (knob control),

switches or combination of both (Voltage controlling resistor), there are other methods too but

we won’t be focusing on them right now. The output of the 1st amplifier is connected to rotary

switch whose other end is connected to various resistances. When switch is closed and rotated

various potential drops can be got across various resistors (series) using potential divider rule.

This drop acts as the input voltage for the 2nd amplifier. Hence the output of 2nd amplifier is

dependent on the input it gets which can be controlled using the switch. We’ll get the maximum

gain if we allow all of the Vout got from the 1st amplifier to appear on the input of the 2nd

amplifier, which is only possible if the resistor value between the 2 stages is least as possible

almost negligible. The total gain of the system would differ as the input voltage to 2nd amplifier

differs. The gain we are referring to is the gain of the whole system that is VOUT by VIN (after

DC coupling).

Op - Amps High Pass Filter:

Operational amplifiers lend themselves to being used for active filter circuits, including a high

pass filter circuit. Using a few components they are able to provide high levels of performance.

The simplest circuit high pass filter circuit using an operational amplifier can be achieved by

placing a capacitor in series with one of the resistors in the amplifier circuit as shown. The

capacitor reactance increases as the frequency falls, and as a result this forms a CR low pass

filter providing a roll off of 6 dB per octave. The cut off frequency or break point of the filter can

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be calculated very easily by working out the frequency at which the reactance of the capacitor

equals the resistance of the resistor. This can be achieved using the formula:

Xc     =     1     /     2     pi     f     C

Op - Amplifier Low Pass Filter Circuit:

Operational amplifiers lend themselves to being used for active filter circuits, including a low

pass filter circuit. Using a few components they are able to provide high levels of performance.

The simplest circuit low pass filter circuit using an operational amplifier simply places a

capacitor across the feedback resistor. This has the effect as the frequency rises of increasing the

level of feedback as the reactive impedance of the capacitor falls. The break point for this simple

type of filter can be calculated very easily by working out the frequency at which the reactance

of the capacitor equals the resistance of the resistor. This can be achieved using the formula:

Xc     =     1     /     2     pi     f     C

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While these operational amplifier circuits are useful to provide a reduction in gain at high

frequencies, they only provide an ultimate rate of roll off of 6 dB per octave, i.e. the output

voltage halves for every doubling in frequency. This type of filter is known as a one pole filter.

Often a much greater rate of rejection is required, and to achieve this it is possible to incorporate

a higher performance filter into the feedback circuitry.

Bypass Capacitor:

A capacitor employed to conduct an alternating current around a component or group of

components. Often the AC is removed from an AC/DC mixture, the DC being free to pass

through the bypassed component.

In practice, most digital circuits such as microcontroller circuits are designed as direct current

(DC) circuits. It turns out that variations in the voltages of these circuits can cause problems. If

the voltages swing too much, the circuit may operate incorrectly. For most practical purposes, a

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voltage that fluctuates is considered an AC component. The function of the bypass capacitor is to

dampen the AC, or the noise. Another term used for the bypass capacitor is a filter cap.

The simplest incarnation of the bypass capacitor is a cap connected directly to the power source

and to ground. This simple connection will allow the AC component of VCC to pass through to

ground. The cap acts like a reserve of current. The charged capacitor helps to fill in any dips in

the voltage VCC by releasing its charge when the voltage drops. The size of the capacitor

determines how big of a dip it can fill. The larger the capacitor, the larger the dip it can handle. A

common size to use is a 0.1uF capacitor. The precise value of a bypass cap isn't very important.

Voltage Matching & Bridging Inputs:

The idea here is to engineer the equipment to have the lowest possible output impedance and a

relatively high input impedance, the difference between them must be at least a factor of ten, and

is often much more. Modern equipment typically employs output impedances of around

150(omega) or below, with input impedances of at least 10k (omega) or above. With the

minuscule output impedance and relatively high input impedance, (the cable impedance can be

disregarded completely in comparison) the full output voltage should be developed across the

input impedance.

Relatively high-impedance inputs such as these are called bridging inputs, and they have the

advantage that several devices can be connected in parallel without decreasing the impedance to

any significant degree, the voltage developed across each input remains high and the source does

not need to supply a high current. (Low impedance is often referred to as loading the output or

circuit, because of the high current it demands.)

Z2 (Zout) is the small internal resistance (source impedance or output impedance) of the first unit

and Z1 (Zin) is the larger external resistance (load impedance, input impedance) of the following

equipment. With resistance R it is always meant the impedance Z. In recording studio technique

the voltage bridging Z1 > Z2 is used for the interconnection in general, which is a voltage divider.

These are great advantages, because it is best for the large frequency bandwidth to be transmitted

and by changing loads, by different cable lengths, and by parallel connection of several devices.

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It keeps the voltage high at the input of the following gear. The impedance matching concept Z1

= Z2 is expected to remain in telecommunications, digital technique, but not in analogue audio.

Buffer amplifier:

A buffer amplifier (sometimes simply called a buffer) is one that provides electrical impedance

transformation from one circuit to another. Two main types of buffer exist: the voltage buffer

and the current buffer.

A voltage buffer amplifier is used to transfer a voltage from a first circuit, having a high output

impedance level, to a second circuit with a low input impedance level. The interposed buffer

amplifier prevents the second circuit from loading the first circuit unacceptably and interfering

with its desired operation. In the ideal voltage buffer in the diagram, the input resistance is

infinite, the output resistance zero (impedance of an ideal voltage source is zero). Other

properties of the ideal buffer are: perfect linearity, regardless of signal amplitudes; and instant

output response, regardless of the speed of the input signal.

If the voltage is transferred unchanged (the voltage gain Av is 1), the amplifier is a unity gain

buffer; also known as a voltage follower because the output voltage follows or tracks the input

voltage. Although the voltage gain of a voltage buffer amplifier may be (approximately) unity, it

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usually provides considerable current gain and thus power gain. However, it is commonplace to

say that it has a gain of 1 (or the equivalent 0 dB), referring to the voltage gain.

A unity gain buffer amplifier may be constructed by applying a full series negative feedback

(figure above) to an op-amp simply by connecting its output to its inverting input, and

connecting the signal source to the non-inverting input (figure below). In this configuration, the

entire output voltage (β = 1) is placed contrary and in series with the input voltage. Thus the two

voltages are subtracted according to KVL and their difference is applied to the op-amp

differential input. This connection forces the op-amp to adjust its output voltage simply equal to

the input voltage (Vout follows Vin so the circuit is named op-amp voltage follower).

The importance of this circuit does not come from any change in voltage, but from the input and

output impedances of the op-amp. The input impedance of the op-amp is very high (1 MΩ to 10

TΩ), meaning that the input of the op-amp does not load down the source or draw any current

from it. Because the output impedance of the op-amp is very low, it drives the load as if it were a

perfect voltage source. Both the connections to and from the buffer are therefore bridging

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connections, which reduce power consumption in the source, distortion from overloading,

crosstalk and other interference.

Construction of the Circuit:

For the input we are using Mobile phone connected to the circuit using 3.5 mm audio wire. The

wire is connected to a 3.5 mm audio jack whose one end is connected to the capacitor and the the

common to the ground. The mobile phone produces sound (AC) which is of mA. We are using

the voltage buffer circuit in which the input impedance is high and output impedance is very low

compared to it, which follows voltage or impedance bridging. The capacitor is there so that no

noise (DC) comes from the source side. Plus the RC circuit we are using here is High band pass

filter which allows only high frequency waves to pass and not the very low frequency ones as

they are or can be noise generated on the source side. We use high resistance connected to Op

amp and capacitor as we want our input to have high reactance so that the source doesn’t have to

exert itself or maybe if it doesn’t have enough current to produce across the input. It is also

helping by protecting the current from going to the ground.

We are using Op – Amp LF356N (JFET) in both our stages of amplification. The main reason

being very low noise from it plus has very high input impedance. The output of the Op – amp is

connected to inverting input having snubber network or RC in parallel between them. The circuit

we constructed is working on the principle of negative feedback. The main principle of the

circuit is it only allows low frequency signals to pass as it is active low pass filter. It stops the

higher frequency signal from passing only the feeble ones are allowed to pass. As they are the

ones which need amplification not the high frequency ones, plus if high frequency signals are

amplified they will become noise as more than audible range, and will cause harm to the circuit.

The working of the RC circuit is dependent upon the cut-off frequency, which is when Xc=R. It

is dependent on time constant RC, which is equal to charging of the capacitor 63.2% of applied

voltage across the given resistor. It also helps in maintaining the AC signals at the input. There is

another resistor connected in series to RC circuit with the other end of it to the ground, whose

main function is to get the potential drop across the inverting input using the potential divider

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rule. The capacitor at the input also blocks DC from going into the source. The electrolytic

capacitor connected to the +15 V power supply is actually a Bypass capacitor. It protects the

circuit from sudden increase in voltage and if the voltage falls it provides voltage stored inside it.

It lets the AC to flow into the earth, it prevents the rippling effect and provides steady source of

voltage for operation of the Op – amps. There is one more connected to -15 V power supply

whose function is absolutely same.

After gain of around 3.3 is achieved from the first op – amp the signal moves towards the second

op –amp. As we are required to construct a selectable gain we have used four resistors and a

potentiometer across which wires are connected to 5 position rotary switch. The input side of the

switch is connected to the output of op – amp one and the 5 positions are connected to wires

across 4 resistors and a potentiometer. The voltage gain is selected by turning the switch and

playing with the potentiometers knob. The voltages from the resistors come across the inputs of

the second Op – amp. This happens using the potential divider circuit using the 5K ohm

potentiometer. An offset potentiometer of 25 - 100K ohms is connected across pins 1 and 5 of

the op –amp for balancing it as per the data sheet also, the output is DC-coupled to directly drive

as a voltage source .Because of this, an offset pot is used to eliminate any DC voltage at the

output with the input shorted to ground. This is not required if the output is capacitive coupled to

the headphones, but DC coupling gives the truest sound reproduction.

The output of second op –amp is connected to a minute resistance just as a safety measure. The

resistance is then in series with another RC (low pass filter) which connects the output to the

inverting input (negative feedback). There is another resistance connected to the input at one end

while being shorted by the wire coming from the RC circuit and the other end going to the

ground. It also works as first op –amp to get the voltage across to the input using the voltage

divider rule. The final output is got by shorting a wire in between the RC circuit and the

resistance just after the output. The gain of the second op – amp is around 3. So this makes the

total amplification of the circuit to 9.9.

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Calculations for the Circuit:

I want gain of the first stage (A1) to be 3.3 = Vout1\Vin1,

I want gain of the second stage (A2) to be 3 = Vout2/Vin2,

So my total gain should be around (A0) = (A1) * (A2) = 9.9

From the gain formulae Av = 1+Rf/Ri,

For Op-Amp stage 1, A1 = 1+ R3/R2 => 3.3 – 1 = >2.3 = R3/R2

(R3 is feedback resistor taken as 10 k ohms so then R2 becomes 4.3 k ohms)

For Op-Amp stage 2, A2 = 1+ (R8+R9)/R13 => 3 – 1 = >2 = (R8+R9)/R13

(R9 is feedback resistor taken as 10 k ohms and R8 is taken as 51 ohms for protecting the

headphone from sudden rise of output if by chance, so then R13 becomes 5.1 k ohms)

For choosing the selectable gain I have chosen 5 position rotary switches, which have 4 different

values of resistor and a potentiometer (5K chosen tuned at 50% which is 2.5 K ohms),

The resistances chosen are R4, R5, R6 and R7

These resistances would be connected in series to the inputs of the Op – Amp stage 2, and

their input would be output of Op – Amp stage 1

R4 would be having maximum resistance as its in series with other resistors which are

below it, and potentiometer would be having least resistance as its directly connected to

Op – Amp 2

We want gains as follows : 1 (R4+R5+R6+R7+PM), 1.6 (R5+R6+R7+PM), 2.8

(R6+R7+PM), 5 (R7+PM) and 10 (Potentiometer) (All values are approximated for

easier calculations)

Using the Potential Divider rule we have,

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Vout2 = Vin1 (2.5)/ (R4+R5+R6+R7+5) *A1*A2 => 1---- (1)

Vout2 = Vin1 (2.5)/ (R5+R6+R7+5) *A1*A2 =>1.6 ---- (2)

Vout2 = Vin1 (2.5)/ (R6+R7+5) *A1*A2 => 2.8 ---- (3)

Vout2 = Vin1 (2.5)/ (R7+5)*A1*A2 => 5 ---- (4)

Vout2 = Vin1(2.5)/(2.5+2.5+No Resistance)*A1*A2 =>10 ---- (5)

We have 4 unknowns and 4 equations,

So, after calculating the value of R7 = 5.1kohms (approx)

So, after calculating the value of R6 = 7.5kohms (approx)

So, after calculating the value of R5 = 13kohms (approx)

So, after calculating the value of R4 = 18kohms (approx)

Then its value in db (R4) = 20 log base 10 (AV) =>20 log base 10 (1) = 0 db

Similarly we can calculate for others the values in db,

R5 = 4 db

R6 = 9 db

R7 = 14 db

Potentiometer 5k = 20db

Capacitors have general values. There wasn’t much need to find it as all the site advice to use

high capacitance capacitor for by - pass capacitor, for RC circuit the values of the capacitors

were already defined (mostly same for various circuits).

Design and Simulation of Circuit:

After deciding which components to be used and how to put them in the circuit, we calculated

the value of each of them to get the amplification as required by us. Now we make our circuit on

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multisim and would test it to see how it works. Below is the circuit we have designed using the

knowledge we have acquired during the research:

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(Selecting the gains using the switch)

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Observation Table:

Input Output Stage 1 Output Stage 2

1.414v 4.643v 707.15mv (lowest)

6.874v (highest)

3.535v 10.359v 1.578v (lowest)

11.578v (highest)

2.121v 6.936v 1.06v (lowest)

9.845v (highest)

These are the values which I got when I connected switch to 0 db (lowest) and 20 db (highest).

The values in between are coming consistent with the highest and the lowest second output stage

gain. I did not show the other values as the highest and the lowest was the main. If they are

consistent the other values follow.

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Benefits of the Circuit:

First of all the capacitor at the start of the circuit prevents any DC noise to coming into the

amplifier circuit from source and getting amplified. Other the RC circuit acts as the high band

pass so doesn’t allow any noise or low frequency sounds which may intervene with the

amplification of the sound. Then we have negative feedback circuit in place which helps in

deciding how much gain do we need we can increase the value of the resistor in the RC circuit to

magnify our gain. The RC circuit in the negative feedback helps only low frequency signal to

amplify not the ones which have high frequency. If they amplify they would produce very loud

sound known as noise. The circuit is DC coupled as the output of stage one operational amplifier

is going directly into the input of stage two operational amplifiers without much change. Only

for controlling the gain we have put various resistances in between with a potentiometer whose

output goes into the input of second op – amp. We select the gain using the rotary switches and

the potentiometer which works on the concept of potential divider. Least the resistance highest

the sound or gain which should have been if there would have been no resistance in between. If

higher the resistance lower the gain got across the second op – amp. In the second op –amp the

offset trimmer helps to adjust the amplifier so that there isn’t any DC occurring at the output as

noise. Then there is RC circuit again at negative feedback which helps in futhur decreasing any

noise.

Practical:

For the practical needed various components namely as,

2 breadboards

1 15V DC power supply

5K ohm potentiometer

100K ohm offset potentiometer

2 3.5mm audio jacks

1 3.5mm audio cable

2 LF356N JFETS

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1 2Pole, 5 Position Rotary Switches

14 -15 Resistances of various ohm values

2 Electrolytic Capacitors of 4.7uf, 20V

3 Capacitors of various values

3 Color wires

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While doing the circuit there were few issues:

The sudden increase of voltage damaged my Op – Amp once, so need to take care that

voltage is at 15V exact not more.

Resistors kept on burning number of times as due to improper connection.

The output was feeble number of times so, had to increase the gain.

If the 1 mega ohm resistance wasn’t used at starting then output was a low.

Conclusion:

After doing this project I learnt a lot about audio circuit plus refreshed everything about op –

amps. Understood how to make a selectable gain switch, working of voltage bridging,

application of RC low pass and high pass filter. Now can use this knowledge for making better

and bigger sound amplifiers. But there are still some things which can be improved in the circuit.

For instance the gain of the op – amps could be increased, output across a bigger speaker could

be achieved, automatic switching action could be put into place and last the left and right both

channels could be made. In end I learnt how the DC coupling takes place between 2 amplifiers

and how can various gains be selected.

References: (All Accessed on 26th March, 2011)

http://www.bcae1.com/opamp.htm

http://www.datasheetarchive.com/LF356%20op-amp%20application-datasheet.html

http://www.tpub.com/neets/book8/30c.htm

http://gilmore2.chem.northwestern.edu/projects/simpson_prj.htm

http://www.radio-electronics.com/info/circuits/opamp_high_pass_filter/

op_amp_highpassfilter.php

http://www.soundonsound.com/sos/jan03/articles/impedanceworkshop.asp

http://www.sengpielaudio.com/calculator-voltagebridging.htm

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http://en.wikipedia.org/wiki/Buffer_amplifier

http://www.bcae1.com/capacitr.htm

http://www2.renesas.com/faq/en/f_opcomp.html

http://www.allaboutcircuits.com/vol_6/chpt_6/10.html

http://zone.ni.com/devzone/cda/ph/p/id/9

http://encyclopedia2.thefreedictionary.com/Direct+coupled

http://www.kpsec.freeuk.com/imped.htm

http://www.allaboutcircuits.com/vol_3/chpt_4/2.html