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Common Emitter Amplifier Linearity v IN v OUT Ideal input/output curve v IN Actual input/output curve v OUT Distortion is proportional to input amplitude

Examples Ideal V OUT time V OUT 0 V OUT with 1mV peak input V OUT with 10mV peak input V OUT with 20mV peak input

Problems with the CE Amplifier Percentage distortion is around 1% per 1mV of input amplitude For inputs bigger than around ±10mV, the output is heavily distorted … also … It only works for a.c. signals Often a differential input is required

Differential Amplifier Features Two identical transistors are used Two input signals, v 1 and v 2 Two available outputs v OUT1 and v OUT2 No capacitors!

Large Signal Analysis

Large Signal Analysis (cont) Also,

Large Signal Analysis (cont) Also, by symmetry:

Operating Regions Q 1 saturated Q 2 cut-off Q 1 cut-off Q 2 saturated Active region To analyse circuit behaviour in the active region we use small signal analysis

Small Signal Analysis NB. All constant voltage and current sources are zero in small signal analysis. (Being constant, they cannot change). But

Small Signal Analysis (cont) Applying Kirchoff’s current law to the ‘v E ’ node gives: Note that as with the common-emitter amplifier:

Small Signal Summary Combining equations gives:

Linearity Ideal V OUT time V OUT 0 CE amp with 20mV input Diff amp with 20mV input

Summary Common-emitter amplifier has its problems Distortion with ‘large’ inputs A.c. response Single input/output Differential amplifier improves in all ways It does need twice as many transistors though … but that isn’t a problem with integrated circuits Next time: Practical design issues Input & output impedances Frequency response and more!