Analog mosfet- large signal and small signal analysis

8/13/2019 Analog mosfet- large signal and small signal analysis

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By Y. Anil Kumar13331D5701

M.TECH (1/1)


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Why MOSFET?

Large signal analysis

Triode

Saturation

Cutoff

Small signal analysis

Common Source

Common Drain

Common Gate


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High power JFETs are not common.

Enhancement mode is not possible in a JFET, i.e. the gate bias has to be below

the source voltage, ( depletion mode) which is awkward for simple powersupply arrangements.

Hence JFET are not suitable for logic gates.

The gate is insulated electrically from the channel, no current flows between

the gate and the channel, no matter what the gate voltage is.

Thus, the MOSFET has practically infinite impedance . This makes MOSFETsuseful for power amplifiers.


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MOS metal-oxide-semiconductor structure.

MOSFET is a four-terminal device: gate (G), source (S), drain (D) and

body (B).

The device size (channel region) is specified by channel width (W) andchannel length (L).

Two kinds of MOSFETs: n-channel (NMOS) and p-channel (PMOS)

devices

The device structure is basically symmetric in terms of drain and source.

Source and drain terminals are specified by the operation voltage.


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Consider the channel between drain and source as a series of resistors, source isgrounded and Vds is connected to drain, so the voltage at each pint along y-axis

is given by VGS- VDS(y)

At the point VDSsat = VGS- Vt , the channel is pinched off at the drain side.Increasing VDSbeyond this value has little effect on the channel shape and iD

saturates at this value.

Triode region: VDS< VGS- VtSaturation region: VDS VGS- Vt


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Drain current in triode region is

Drain current in saturation region is given by making VDS =VGS- Vth

Transconductance gmis given by


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Cutoff : VGS= 0 Sub threshold : 0 < VGS< Vt

Sub threshold region is useful in low power, low frequency, and low speed

applications

Example : Biomedical applications like heart beat measurement, blood

pressure measurement


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The expression we derived for saturation region assumedthat the pinch-off point remains at the drain and thus iD

remains constant.

In reality, the pinch-off point moves slightly away from

the drain: Channel-width Modulation


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Recall that Drain-Body and Source-Body diodes should be reversed biased. Weassumed that Source is connected to the body (VSB= 0) and VDS= VDB> 0

In a chip (same body for all NMOS), it is impossible to connect all sources to the

body (all NMOS sources are connected together.

Thus, the body (for NMOS) is connected to the largest negative voltage (negative

terminal of the power supply).

Doing so, changes the threshold voltage (called Body Effect)


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Low frequency model: At low frequencies the capacitors can be neglected


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Common source configuration serves as the gain stage. The disadvantage is high

output impedance.

BTW, to be a good voltage amplifier, we want Rin large, Ro small and Avo large.Feedback (Rs)lowers the gain of the stage, compared to a pure common-source

configuration. However, the benefit is an increased linearity and better high

frequency behavior.

Common drain (Source Follower) configuration acts as a voltage buffer. It provides

no gain, but has low output impedance. It is typically the last stage in a multi-stage

amplifier.


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This amplifier provides gain and is useful when a specific (low) Rin is required.

This is, e.g., the case when the impedance needs to be matched, as with

transmission lines (e.g. to 50 ). Another application of the CG configuration is

that it acts as a current buffer (current gain close to unity, small Rin, large Rout).


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Analog mosfet- large signal and small signal analysis