Common Emitter Amplifier To analysis the circuit Determine

Small Signal Analysis: Voltage Gain As before:

Input and Output Impedance • Unlike the op-amp, transistor amplifiers have significant output impedances

Input Impedance i. IN i. RB i. B • Input impedance, r. IN, is

Output Impedance • One way to measure r. OUT is: – Short the input

Output Impedance (cont) Applying Kirchoff’s current law: By Ohm’s law:

Coupling Capacitors • Capacitor COUT is needed to remove the d. c. component of

Cut-Off Frequency Cut-off frequency, or – 3 d. B point, is when the gain

COUT • For the lower cut-off frequency calculation to be valid, COUT should still

Emitter Capacitor For the highest voltage gain, But, v. BE v. IN v. E

Emitter Capacitor (cont) For CE to not interfere at f. C: Where, v. BE

Summary • In the context of the common-emitter amplifier we have covered: – Small

Slides: 15

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Common Emitter Amplifier • To analysis the circuit: – Determine quiescent conditions – Calculate transconductance – Calculate small signal performance • • Voltage Gain Input Impedance Output Impedance Cut-off frequency

Quiescent Conditions

Small Signal Analysis: Voltage Gain As before:

Input and Output Impedance • Unlike the op-amp, transistor amplifiers have significant output impedances and finite input impedances – RIN can be comparable with the source resistance of the input signal – ROUT can be comparable with the load resistance

Input Impedance i. IN i. RB i. B • Input impedance, r. IN, is the ratio of the small signal input voltage and the small signal input current

Input Impedance (cont) i. IN i. RB i. B

Output Impedance • One way to measure r. OUT is: – Short the input to 0 V – Output now looks like just r. OUT

Output Impedance (cont) Applying Kirchoff’s current law: By Ohm’s law:

Coupling Capacitors • Capacitor COUT is needed to remove the d. c. component of the collector voltage • Capacitor CIN is needed to allow the base voltage to be offset from 0 V • In both cases this is known as coupling • Both capacitors are chosen to look like short circuits at operating frequencies • Their reactance will, however, become significant at low frequencies

Equivalent Circuit

Cut-Off Frequency Cut-off frequency, or – 3 d. B point, is when the gain of the amplifier falls by a factor of Ö 2 If the cut-off frequency, f. C, is specified and r. IN has been calculated: NB. This assumes that COUT still looks like a short circuit

COUT • For the lower cut-off frequency calculation to be valid, COUT should still look like a short circuit at f. C • Typically, choose:

Emitter Capacitor For the highest voltage gain, But, v. BE v. IN v. E where, Also,

Emitter Capacitor (cont) For CE to not interfere at f. C: Where, v. BE v. IN v. E To make sure, choose, NB. Use r. E (=VT/IC) not RE for this calculation!

Summary • In the context of the common-emitter amplifier we have covered: – Small signal analysis – Mutual conductance – Input/output impedance – Coupling capacitor requirements and cut-off frequencies • Next time: – Applying the same principles to the differential amplifier – It’s actually a much easier circuit to analyse – honest! – Make sure you’re happy with the fundamentals by then!