Microelectronic Circuits. Feedback Amplifiers. Slide 1. Lecture on Microelectronics Circuits. BITS Pilani, Dubai Campus. Dr. Vilas

Transcription

1 Microelectronic Circuits Feedback mplifiers Slide 1

2 General Structure of Feedback Comparison Circuit / Mixer x o = x i ; x f = b x o ; x i = x s - x f ; f = (x o /x s ) = / (1+b). lso, x f = bx s / (1+b) i.e., if b >>1 => x f = x s under the conditions of Large amount of negative feedback Slide 2

3 General Structure of Feedback If feedback is such that it reduces x i then it is called Negative Feedback Here, b is defined as LOOP GIN Closed Loop Gain, f is always smaller than open loop gain by the quantity (1+b), which is called the mount of Feedback / the Desensitivity factor (D). Properties: Gain De-sensitivity Bandwidth Extension Reduction in Nonlinear Distortion Substantial reduction in Frequency Distortion Reduction in Noise Improved Terminal Characteristics: R i & R o Slide 3

4 Basic Topology of Negative Feedback mplifier voltage-mixing voltage-sampling (series shunt) topology current-mixing current-sampling (shunt series) topology voltage-mixing current-sampling (series series) topology current-mixing voltage-sampling (shunt shunt) topology Slide 4

5 Series-Shunt Feedback mplifier (Voltage amplifier) f = V V o s = 1 b ideal structure equivalent circuit. Slide 5

6 Series-Shunt Feedback mplifier (Voltage amplifier) CG amplifier with a fraction V f of the output voltage feedback to the gate through voltage divider (R 1, R2) Slide 6

7 Shunt-Series Feedback mplifier (Current amplifier) Slide 7

8 Shunt-Series Feedback mplifier (Current amplifier) The feedback current I f that flows through R F is subtracted from I s at the source node I i = I s - I f Increase in I s causes I i to increase and the drain voltage of Q1 will also increase. Slide 8

9 Series-Series Feedback mplifier (Transconductance amplifier) f = I V o s = 1 b Rof = ( 1 b ) Ro Rof = ( 1 b ) Ro Slide 9

10 Series-Series Feedback mplifier (Transconductance amplifier) Slide 10

11 Shunt-Shunt Feedback mplifier (Transresistance amplifier) R f R of if V I o s Ro = 1 b = 1 b Ri = 1 b Slide 11

12 Shunt-Shunt Feedback mplifier (Transresistance amplifier) We want to analyze the circuit of Figure to determine the small-signal voltage gain Va/Vs, the input resistance Rin, and the output resistance Rout = Rof. The transistor has b = 100. Slide 12

13 Shunt-Shunt Feedback mplifier (Transresistance amplifier) Slide 13

14 Comparison Slide 14

15 Cascode mplifier Cascoding refers to the use of a transistor connected in the common gate configuration to provide the current buffering for the common source amplifying transistor. M1 is mplifying Transistor and M2 is a common gate (CG) with dc bias. R out g r r m2 O2 O1 v = G m R out v g r g r m1 O1 m2 O2 Slide 15

16 Cascode mplifier Cascoding refers to the use of a transistor connected in the common Base configuration to provide the current buffering for the common Emitter amplifying transistor. R out g r ( r r 2 ) m2 O2 O1 v = G m R out v g ( gm2ro 2) ( ro 1 r 2) m1 Slide 16

17 Cascode MOS Mirror In ddition to the diode connected transistor Q1, another diode connected transistor Q4 is used to provide a suitable bias to the base of the cascode transistor Q3. Slide 17