C0387pt5

7/28/2019 C0387pt5

1/10

and cheaper than anyone else. A nice benefit of this type of anticipative designis that when you are asked to develop a similar product, you have all the pieces

in place. You simply add or subtract the required feature and are done with it.

Finally, best of all, MAMA will not drive you berserk!

One Last Word of Caution

It is possible to go too far with this philosophy. Dont try to make your design

so universal that it comes at the expense of getting the product to market oradds so much cost for all the options that it is no longer viable. Remember,

there is also a chance you will never use the option you built in, so choose

wisely, young Jedi.27

Thumb Rules

Read the datasheet. Consider tolerances.

Know the environment.

Test, check, and retest.

Make your own list of Thumb Rules or design guidelines.

Do research on standards or guidelines that exist for your product.

MAMA can be frustrating.

Modularize the design.

Anticipate changes.

Dont go too far.

SOME OF MY FAVORITE CIRCUITS

Every engineer has their favorite batch of circuits, and Im no exception. Thereare tons of circuit cookbooks out there that show how to implement no end of

cool features. There are so many that you could spend all your time searching

them and never getting anything done. I suggest you develop your own favorite

basic circuits that you know well and intuitively understand. This is simply an

27Do Jedi mind tricks work in the cooperate world? I think so. Now, that is a cool idea for a

book. Email and let me know if you would buy it. If I get enough responses I definitely willexplore that idea!

Some of My Favorite Circuits 193

08_Y506_Ch04.indd 19308_Y506_Ch04.indd 193 10/17/2008 5:01:03 PM10/17/2008 5:01:03 PM

7/28/2019 C0387pt5

2/10

extension of the Lego philosophy that we discussed way back at the beginningof the book. Here are a few of my favorites. These are in addition to all the

circuits I have used as examples up to this point. One reason they make such

good examples is that they are so useful.

Hybrid Darlington Pair

Cool application note: using two transistors to switch a signal level Vcc PNP

switched by NPN.

Figure 4.33 shows a handy circuit that switches a higher-level voltage with a

lower-level one. Say, for example, you have a micro with a 5 V output and you

need to drive a 12 V load. For a reason you cant change, you have to switch

the Vcc leg. In this circuit you turn on one transistor with a 5 V signal, which in

turn activates the other transistor, switching the higher voltage to the load.

This works because the transistors are current driven; when you shut off the

current flow to the PNP transistor, it shuts off regardless of the voltage. Another

plus is that this circuit has Darlington-like properties without one of the down-

sides. You wont need a lot of current to the input to switch the output and,

unlike a traditional Darlington pair, the voltage drop across the output is much

smaller. You dont have two series base junctions to contend with at the out-

put. If you still dont follow, try a little ISA28 on it.

12 V

Low

Volt

In

Load

FIGURE 4.33

VccPNP switched by NPN.

28Intuitive signal analysis (ISA). I still hope to someday cement my legacy in an acronym.

CHAPTER 4 The Real World194


7/28/2019 C0387pt5

3/10

DC Level Shifter

This is really the high-pass filter that we have already studied but with a slight

twist, as shown in Figure 4.34. Instead of ground, we hook the resistor to a ref-

erence voltage. Since DC has a frequency of zero, only the AC component will

pass and in the process a DC bias will be applied to the signal. Make sure thatyou dont size the cap and resistor so that the signal you want is attenuated.

Virtual Ground

Using the voltage divider as a reference, the op-amp becomes a voltage source

with the output matching the voltage at the dividersee Figure 4.35. This can

VoOriginalSignal

New DCBias Voltage

FIGURE 4.34

Change the DC bias on an AC signal.

Virtual

Ground

FIGURE 4.35Create a ground at any level you want.

Some of My Favorite Circuits 195


7/28/2019 C0387pt5

4/10

be very useful when you are trying to handle AC signals with only a single-ended supply circuit.

Voltage Follower

As Figure 4.36 shows, this one is mighty useful when youre trying to measure

a signal that is easily affected by load. ViVo, but, best of all, Vi isnt loaded at

all, thanks to the buffering effect of the op-amp.

AC-Only Amplifier

Figure 4.37 shows another great circuit that works nicely in amplifying AC sig-

nals with a single-ended supply. It also has the benefit of not amplifying any

DC signal components, keeping things like DC offsets from making your sig-

nal rail. This happens because of the cap in the feedback circuit. Since the cap

only passes AC current, DC signals see that point as disconnected. When the

resistor to ground is disconnected, the op-amp acts like the voltage follower in

the previous circuit.

Vin

Vo

FIGURE 4.36

Voltage follower.

Vcc

Vo

2.5V

Virtual

Ground

Vi

FIGURE 4.37AC-only amplifier.



7/28/2019 C0387pt5

5/10

Inverter Oscillator

I saw this in the back of a data book years ago; I think it was a Motorola logic

data book. This was way back before the Internet. You used to have to turn

actual pages to find this stuff! The way it works is based on the fact that theSchmidt trigger inverter has hysteresis built into the input (Figure 4.38). This

makes the output stick at a high or low level till the cap on the input charges

to the threshold voltage that trips the inverter. Output flips and everything goes

in the other direction, repeating indefinitely. Adding some diodes to the charge

and discharge path can affect the duty cycle of the output.

Constant Current SourceUsing negative feedback, the op-amp tries to maintain the voltage drop across

R input. Even if the resistance of the load changes, the drop across R input stays

the same. According to Ohms Law, keepingR and Vthe same will keep current

the same, toosee Figure 4.39 on next page. Remember, though, this current

control has operational limits; it can only swing the output voltage so far to

compensate for load variance. Once these limits are reached, the current regula-

tion can no longer exist.

GET YOUR OWNHERE ARE A FEW

I have just a few favorite circuit concepts. Get your own and know them well.

You will be better served knowing a few circuit concepts inside-out than know-

ing thousands superficially.

Following this advice, several readers of the first edition sent in some of their

favorite circuits. Without further ado, they are presented next.

R

C 74HC14

FIGURE 4.38

Schmidt trigger oscillator.

Get Your OwnHere are a Few 197


7/28/2019 C0387pt5

6/10

On/off

Switch

9V

Battery

NC

5V Relay

Coil

Shock

PadsFIGURE 4.40

Toy shocker circuit.

Vcc

Load

R Input

Current changesInversely to voltage

making feedbacknegative

I

Voltage heredetermines

current at load

FIGURE 4.39

Voltage-controlled constant current source.


Steve Petersen sent in the circuit shown in Figure 4.40, saying something about

being fun for parties and the potential to add a delay circuit to really surprise


7/28/2019 C0387pt5

7/10

someone29 when they picked up whatever interesting device the circuit wasembedded in.

Travis Hayes sent in the diagram of a slick little circuit, as shown in Figure 4.41,

that uses the inverter oscillator from my list to drive a voltage doubler circuit.

He said it was a pretty slick and inexpensive way to get a higher voltage for an

LCD he was using. Id have to agree!

0.1 F

0.01 F

Vcc

0.1 F

1.5 K

V Out

FIGURE 4.41

Inverter-driven voltage doubler.

29I hereby claim no responsibility whatsoever for anyone out there hurting themselves using

a design they found in this book when I took the effort in this footnote to say Dont try this

at home. We book writers are professionals and know how to do a practical joke without

really hurting anyone, at least not too badly!

30This circuit was published in the Q&A column of Electronics Now some time in

the late 1990s when I was writing that column for the magazine. The publisher has givenpermission to republish it elsewhere.


Alan Tyger just might be as big a fan of op-amps as I am; he sent in the circuitdiagram shown in Figure 4.42 on the next page; it uses just such a device to

store a piece of information.

Michael Covington30 sent in the cool circuit shown in Figure 4.43 on the next

page; it combines the fun of remote controls with a laser pointer. The 555 acts

as a memory cell (not unlike Alans circuit), but this one has the added bonus

that you use a laser to control it. How cool is that! I dont know any engineer

who doesnt like lasers, and Im pretty sure they all have to control the remotewhen they are home watching TV.


7/28/2019 C0387pt5

8/10

CV

TH

TR

RS

V

V

OUT

4

5

6

2

8

3

1

IC1

TLC555

DS7

R11k

R21k

TwoCdS

photocells(same type)

D11N4001

C10.1

F

Hit one or the other photocell with a laser pointer to change states. Supply

voltage is not critical (5 to 15 V depending on requirements of relay).

Note: Either R1 or R2 (not both) can be omitted to make up for imbalancebetween the photocells and provide better performance in bright-light

conditions.

Relay

FIGURE 4.43Laser light switch.

10K

10K3.3K 10K

1K

Reset

Set

FIGURE 4.42

Flip-flop memory op-amp.



7/28/2019 C0387pt5

9/10

Mike Angeli sent in this cool circuit in Figure 4.44. He said he used it to position

a load using a potentiometer feedback (thus the high impedance requirement).

Sam Nay sent in the circuit shown in Figure 4.45, saying he was always fas-

cinated by the ability to transmit data without wires. Ill bet he hooks up the

laser-controlled switch that we saw just moment ago. Also, I happen to know

Set toHigh

Level

Signal

Simply Inverts

the Signal

Optional

Optional

Set to

1/2 Vcc

V Out 1

V Out 2

Set to

LowLevel

FIGURE 4.44

High-impedance window comparator.


Switch Vcc2

Signal

Output

FIGURE 4.45

Optical signaltransmission.


7/28/2019 C0387pt5

10/10

of a secret circuit that I am not at liberty to disclose that uses a variation ofoptical circuits not that different from this one to take biometric readings. Bet

you wish I could show you that one, dont you!?

Finally, Mourly Thov sent in the circuit shown in Figure 4.46. He said he just

thought it was a slick way to change the DC voltage (and have some power

capacity, which could be an issue with the one Travis sent in), so if you find your-

self in need of a different voltage that can move some current, try an idea like

this one.

DC Out

FIGURE 4.46

Isolated DC-DC converter.

Thumb Rules

Keep your own cookbook of cool circuits.

Learn them well.


30Look it up in the index at the back of this book. Ill bet you know some RSPs too!

On a final note, I have to say that from my communication with these engi-

neers, I think they all fall in the RSP30 category. Then again, maybe that is just

because they emailed me and really liked the first edition of this book. Either

way, I thank them for their submissions and completely absolve myself from

any responsibility for these circuits actually working. I hope they bring you

luck and help you to fill up a notebook with your favorite circuits.


Documents

C0387pt5