Electronic PRINCIPLES

Transcription

1 MALVINO & BATES Electronic PINCIPLES SEVENTH EDITION

2 Chapter 20 Linear Op-Amp Circuits

3 Topics Covered in Chapter 20 Inverting amplifier circuits Noninverting amplifier circuits Inverter/noninverter circuits Differential amplifiers Instrumentation amplifiers

4 Topics Covered in Chapter 20 (Continued) Summing amplifier circuits Current boosters Voltage-controlled current sources Automatic gain control Single-supply operation

5 Inverting amplifier circuits One of the most basic circuits Voltage gain equals the ratio of the feedback resistance to the input resistance Example circuits include: ac coupled and adjustable bandwidth

6 ac coupled inverting amplifier f C 1 1 C 2 v in A V = - f 1 f 2 = f unity f 1 +1 L 1 f c1 = 2π1 C 1 1 f c2 = 2πL C 2

7 Adjustable bandwidth f 1 v in A V B = f - f 1 f 2 = Bf unity f

8 Noninverting amplifier circuits One of the most basic circuits Advantages include: stable voltage gain, high input impedance, and low output impedance An example circuit is an ac coupled noninverting amplifier

9 ac coupled noninverting amplifier C 1 C 2 v in 2 f L B dc = f c3 = 2π1 C 3 C 3

10 Inverter / noninverter circuit When an input signal drives both inputs, inverting and noninverting amplification result at the same time Total voltage gain equals voltage gain of the inverting channel plus voltage gain of the noninverting channel

11 JFET controlled reversible gain vin 0 V Low gate: A V = 1 High gate: A V = 1 V GS(off)

12 Adjustable gain of ± 1 vin

13 Phase shifter v in C t φ = -2 arctan f/f c φ

14 Differential amplifier 1 2 v in(cm) v in A V = v in(cm) 1 2 CM limiting factors are the op amp itself and the tolerance of the resistors. Ideally: ±2 < A V(CM) < ±4 1 = 1 2 = 2

15 Wheatstone bridge circuit +V Transducer

16 Wheatstone bridge The differential output signal is small. The common-mode output signal is large. Differential amplifiers are a good match. Input transducers convert nonelectrical quantities into an electrical quantity. Examples: photoresistor: light to resistance thermistor: temperature to resistance

17 Instrumentation amplifiers Differential amplifiers optimized for dc performance Large differential voltage gain High CM Low input offsets Low temperature drift High input impedance

18 v in(cm) Instrumentation amplifier 1 v in G 1 v in(cm) A V = A V(CM) = ±2 G

19 Monolithic instrumentation amplifiers Use laser-trimmed resistors for high performance. esistor G is external and is selected to set the differential gain. esistor G can be split into two devices for guard driving (bootstrapping the cable shield to the common-mode potential).

20 Guard driving. G 2 G 2 Instrumentation amplifier Guard voltage (common-mode voltage)

21 Summing amplifier circuits Examples include: summing on both inputs and D/A converter D/A converter is used in multimeters to measure voltages, currents, and resistances

22 Summing amp with inverting and noninverting inputs v 1 1 f v 2 2 v 3 3 v 4 5 4

23 D/A converter v 3 v 2 v v in v 0 8 N = 4 = -(v v v v 0 ) Possible combinations = 2 N = 2 4 = 16

24 Current booster Used when short-circuit output current of one amp is too low Used on the output side of the circuit Typically, a transistor whose base current is supplied by the op amp Output current increased by the β factor

25 Unidirectional current booster +V CC v in 741 β dc = I SC = 25 ma 1 L I max = β dc I SC = 100(25 ma) = 2.5 A

26 Bidirectional current booster 2 +V CC 1 v in L -V EE

27 Voltage controlled current sources Input voltage controlled current sources can be configured Loads can be floating or grounded Load currents can be unidirectional or bi-directional The Howland current source is a bidirectional voltage controlled current source

28 AGC circuit v in Voltage-controlled resistance (r ds ) 6 +V AGC 3 4 As the signal level increases, V AGC goes more positive. As the JFET r ds drops, the input signal is attenuated. -V EE

29 Single-supply operation Op amps typically use dual supplies. However some applications are better suited to a single supply. Some op amps are optimized for single supply operation.

30 Single-supply inverting amplifier 2 C +V 1 CC 1 C 2 v in C 3 L +V CC 2