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Practical Differential Amplifier Design We’ve discussed Large signal behaviour Small signal voltage gain Today: Input impedance Output impedance Coupling & biasing D.C. effects Comparisons with the common-emitter amplifier

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Input and Output Impedances An equivalent small signal circuit of a differential amplifier can be drawn as:

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Input Impedance During the small signal analysis, it was shown that: But,

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Output Impedance Applying Kirchoff’s current law: By Ohm’s law: NB. Same result as common emitter amplifier

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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

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Constant Current Source Current, I, should be constant regardless of varying V E In practice, during small signal operation V E doesn’t 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

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Current Mirror V BE V BE is unknown, but should be around 0.5 V So, Exact equilibrium value of V BE is set by negative feedback and can be found from:

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Current Mirror (cont) V BE is identical for both transistors and So, But we know,

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Practical Amplifier with Coupling

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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

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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

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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

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Applications Differential inputs and outputs Useful when negative feedback is required in a multi-stage amplifier Also useful for balanced signals Transmitter Noisy Channel Noisy received signals Difference Amp Output

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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

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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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