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Electronic Circuits Laboratory EE462G Simulation Lab #9. The BJT Differential Amplifier. Differential Amplifier. The object of the differential amplifier is to amplify the difference between V in1 and V in2 for output V out . V CC. R C1. R C2.

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differential amplifier
Differential Amplifier

The object of the differential amplifier is to amplify the difference between Vin1 and Vin2 for output Vout.

VCC

RC1

RC2

In many applications VEE = -VCC. This can be obtained in the lab by setting the one negative and positive terminals of the dual power supply to earth ground and then set the power supply for the positive connection to the circuit to VCC and the negative connection to VEE.

+ Vout -

+

Vout1

_

+

Vout2

_

Q1

Q2

Vin2

Vin1

RE

VEE

differential amplifier inputs
Differential Amplifier Inputs

The ideal differential amplifier suppresses the common mode input given by:

VCC

RC1

RC2

+ Vout -

+

Vout1

_

+

Vout2

_

and amplifies differential mode input given by:

Q1

Q2

Note that the input to this system is uniquely determined by the (Vin1, Vin2) pair or the (Vicm, Vidm) pair. There are several types of gain that can now be described for this amplifier.

Vin2

Vin1

RE

VEE

differential amplifier gain
Differential Amplifier Gain

The gain between Vidm and Vout1 is described as the single-ended ideal differential gain given by:

VCC

RC1

RC2

+ Vout -

+

Vout1

_

+

Vout2

_

A similar gain is described for Vout2:

Q1

Q2

Vin2

Similar gains can be described for the common mode input:

Vin1

RE

VEE

differential amplifier gain5
Differential Amplifier Gain

The single ended output can be computed from the previously defined gains using superposition given by:

An important performance measure for differential amplifiers is the common-mode rejection ratio (CMRR) given by:

small signal model
Small Signal Model

vout1

vout2

Circuits I analysis methods can be applied to compute the previously described gains and CMRR in terms of circuit parameters.

RC1

RC2

ib2

ib1

r

r

ib1

ib2

vin2

vin1

RE

Where are the Q1 and Q2 transistor nodes on this model?

large signal model

RC2

RC1

- Vout1 +

+ Vout2 -

VCC

Ib1

Ib2

VBE

VBE

VEE

Ib1

Ib2

Vin1

RE

Vin2

Large Signal Model

Circuits I analysis methods can be applied to compute the quiescent points in terms of circuit parameters.

Where are the Q1 and Q2 transistor nodes on this model?

crude op amp
Crude Op Amp
  • A simple Op Amp can be create from the differential amp. Most Op Amps have additional stages to buffer the output (See http://www.williamson-labs.com/480_opam.htm ).