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Practical Differential Amplifier Design

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An equivalent small signal circuit of a differential amplifier ... Applying Kirchoff's current law: By Ohm's law: NB. Same result as common emitter amplifier ... – PowerPoint PPT presentation

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Title: Practical Differential Amplifier Design


1
Practical Differential Amplifier Design
  • Weve discussed
  • Large signal behaviour
  • Small signal voltage gain
  • Today
  • Input impedance
  • Output impedance
  • Coupling biasing
  • D.C. effects
  • Comparisons with the common-emitter amplifier

2
Input and Output Impedances
An equivalent small signal circuit of a
differential amplifier can be drawn as
3
Input Impedance
During the small signal analysis, it was shown
that
4
Output Impedance
NB. Same result as common emitter amplifier
5
Coupling and Biasing
  • Input and output coupling capacitors may be
    required to remove d.c. bias voltages
  • If input coupling capacitors are used, a d.c.
    bias current path to the transistors bases must
    be established
  • Extra base resistors accomplish this
  • These will appear in parallel with the input
    impedance

6
Constant Current Source
  • Current, I, should be constant regardless of
    varying VE
  • In practice, during small signal operation VE
    doesnt vary by more than a fraction of a volt so
    a resistor is a good approximation (as in the lab
    experiment)
  • For a better approximation, a current mirror is
    often used

7
Current Mirror
VBE is unknown, but should be around 0.5 V
VBE
8
Current Mirror (cont)
VBE is identical for both transistors and
So,
9
Practical Amplifier with Coupling
10
Non-Ideal D.C. Effects
  • If operation down to d.c. is required, the
    coupling components are omitted
  • This leads to some effects that are peculiar to
    d.c. operation
  • Offset Voltage
  • Bias Current

11
Offset Voltage
  • With zero differential input, the collector
    currents and, therefore, the collector voltages
    should be identical
  • This assumes that
  • The transistors are identical
  • The loads are also identical
  • In practice, loads will vary and the quiescent
    conditions will not be perfectly symmetrical
  • There will be an offset voltage between the
    actual output and the ideal assumption

12
Bias Current
  • In order to bring the transistors into the active
    region, a small d.c. base bias current is
    required
  • This d.c. current must be supplied by the signal
    source
  • This is a separate issue to the current drawn by
    the input impedance
  • Note that bias current and offset voltage effects
    are identical to those observed with op-amps

13
Applications
  • Differential inputs and outputs
  • Useful when negative feedback is required in a
    multi-stage amplifier
  • Also useful for balanced signals

14
Comparisons with CE Amp
  • Common Emitter Features
  • One transistor required
  • Single input, single output
  • Maximum input amplitude for linear operation
    around 1 mV
  • High gain possible with high input impedance
  • Differential Features
  • At least two transistors required
  • Differential input, differential output
  • Maximum input amplitude for linear operation
    around 50 mV
  • Reduced gain possible with high input impedance

15
Multi-Stage Amplifiers
  • With both common-emitter amplifiers and
    differential amplifiers, a design compromise must
    be struck between
  • Voltage gain
  • Input impedance
  • Output impedance
  • Simultaneously achieving specified requirements
    may not be possible using a single amplifier
  • Solution cascade more than one amplifier in
    series
  • More on this next time
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