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Lecture 8 - 1 Lecture 8 Introduction to electronic analog...

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Lecture 8: Introduction to electronic analog circuits 361-1-3661 1 Our aim is to study the effect of negative feedback on the small-signal gain and the small-signal input and output impedances of the single-transistor circuits. Our study will be based on generic functional models of the circuits (see Fig. 1). 6.1. Single-transistor circuits with no feedback Let us start from analyzing the small-signal gain of a circuit with no feedback [see Fig. 1(a)] OL A G A = , (1) where G is the small-signal input transmission s s s G ε , (2) s s is the signal source value, s ε is the signal at the control port of the dependent source of the transistor model, and A OL is the small-signal open-loop gain ε s s A o OL . (3) Equation (1) shows that the small-signal gain A is directly proportional to the small-signal open-lop gain A OL . This may be a serious disadvantage because A OL depends on the transistor small-signal parameters, which are very sensitive to the transistor technology and temperature. On the other hand, this can be an advantage if the maximum gain is required and its exact value is not important. We will also see later when studying positive feedback, that the circuit with no feedback is always stable provided its G and A OL gains are stable. In the next course on electronic analog circuit, you will see as well that adding negative feedback can limit the frequency range of the circuit. 6.2. Single-transistor circuits with feedback Let us now find the closed-loop gain of a single-transistor circuit with feedback [see Fig. 1(b)] G A OL L s ε s o A OL L s ε s o Σ β s s Σ β fwd (a) (b) s ε A OL s ε A OL β s s G β fwd s s G h fe i b g m v be r o h ie R C v o v s i b i o R in v be r s s s A OL = − g m ( R C ||r o ) G = R in r s +R in Example G s s s s Fig. 1. Functional (block) diagrams of electronic circuits (a) without and (b) with feedback. D DSF A G D RR A G G A A G s G s G A s s A s s s A OL OL fwd OL OL s fwd s OL OL s o CL + = + + = + + = + + = 1 1 β β β β ε ε ε , (4) where β is the small-signal feedback transmission of the feedback network, β fwd is the small-signal feedforward transmission of the feedback network, 6. Negative Feedback in Single- Transistor Circuits © Eugene Paperno, 2008
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Lecture 8: Introduction to electronic analog circuits 361-1-3661 2 RR A OL β (5) is the return ratio, D G β fwd (6) is the small-signal direct transmission, and DSF 1+ A OL β (7) is the desensitivity factor or the amount of feedback.
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