PHYS225 Lecture 10. Electronic Circuits

Transcription

1 PHYS225 Lecture 10 Electronic Circuits

2 Last lecture Operational Amplifiers Many applications Use feedback for control Negative feedback Ideal case rules Output is whatever is needed to make inputs equal Inputs draw no current Assumes negative feedback

3 Follower (The Art of Electronics, Horowitz and Hill, 2 nd Ed.)

4 Inverting Amplifier (The Art of Electronics, Horowitz and Hill, 2nd Ed.)

5 Non-Inverting Amplifier (The Art of Electronics, Horowitz and Hill, 2 nd Ed.)

6 Summing Amplifier R 1 R f V 1 V 2 R 2 + V out Much like the inverting amplifier, but with two input voltages inverting input still held at virtual ground I 1 and I 2 are added together to run through R f so we get the (inverted) sum: V out = R f (V 1 /R 1 + V 2 /R 2 ) if R 2 = R 1, we get a sum proportional to (V 1 + V 2 ) Can have any number of summing inputs we ll make our D/A converter this way

7 Differencing Amplifier R 2 V R 1 V + R 1 + V out R 2 The non-inverting input is a simple voltage divider: V node = V + R 2 /(R 1 + R 2 ) So I f = (V V node )/R 1 V out = V node I f R 2 = V + (1 + R 2 /R 1 )(R 2 /(R 1 + R 2 )) V (R 2 /R 1 ) so V out = (R 2 /R 1 )(V V ) therefore we difference V and V

8 Differentiator (high-pass) R V in C + V out For a capacitor, Q = CV, so I cap = dq/dt = C dv/dt Thus V out = I cap R = RC dv/dt So we have a differentiator, or high-pass filter if signal is V 0 sin t, V out = V 0 RC cos t the -dependence means higher frequencies amplified more

9 Low-pass filter (integrator) C V in R + V out I f = V in /R, so C dv cap /dt = V in /R and since left side of capacitor is at virtual ground: dv out /dt = V in /RC so and therefore we have an integrator (low pass)

10 Current Source (Student Manual for The Art of Electronics, Hayes and Horowitz, 2 nd Ed.)

11 Improved Current Source OR (BJT for Q 1 ) (MOSFET for Q 1 ) (The Art of Electronics, Horowitz and Hill, 2 nd Ed.) (Student Manual for The Art of Electronics, Hayes and Horowitz, 2 nd Ed.)

12 Improved Current Source (The Art of Electronics, Horowitz and Hill, 2 nd Ed.)

13 Current-to-Voltage Converter Photodiode as input current source I (The Art of Electronics, Horowitz and Hill, 2 nd Ed.) (Student Manual for The Art of Electronics, Hayes and Horowitz, 2 nd Ed.)

14 Photodiode (Introductory Electronics, Simpson, 2 nd Ed.)

15 Current-to-Voltage Converter Photodiode as input current source (Student Manual for The Art of Electronics, Hayes and Horowitz, 2 nd Ed.)

16 Application of Photometer Circuit (Scope sweep rate of 0.5 s/div works best) (Student Manual for The Art of Electronics, Hayes and Horowitz, 2 nd Ed.)

17 Ideal Current Meter Feedback resistor Need current-limiting resistor here I in 0 V I meter 500 W (Student Manual for The Art of Electronics, Hayes and Horowitz, 2 nd Ed.)

18 Summing Amplifier V 1 V 2 V 3 V 1 V 2 V 3 (Student Manual for The Art of Electronics, Hayes and Horowitz, 2 nd Ed.)

19 Push-Pull Buffer Crossover distortion evident (Student Manual for The Art of Electronics, Hayes and Horowitz, 2 nd Ed.) Crossover distortion eliminated (The Art of Electronics, Horowitz and Hill, 2 nd Ed.)

20 Hiding Distortion Consider the push-pull transistor arrangement to the right an npn transistor (top) and a pnp (bot) wimpy input can drive big load (speaker?) base-emitter voltage differs by 0.6V in each transistor (emitter has arrow) input has to be higher than ~0.6 V for the npn to become active input has to be lower than 0.6 V for the pnp to be active There is a no-man s land in between where neither transistor conducts, so one would get crossover distortion output is zero while input signal is between 0.6 and 0.6 V in V + V out crossover distortion

21 Stick it in the feedback loop! V + V in + out V input and output now the same By sticking the push-pull into an op-amp s feedback loop, we guarantee that the output faithfully follows the input! after all, the golden rule demands that + input = input Op-amp jerks up to 0.6 and down to 0.6 at the crossover it s almost magic: it figures out the vagaries/nonlinearities of the thing in the loop Now get advantages of push-pull drive capability, without the mess

22 Restrictions on the Golden Rules Golden rules I and II are followed only if: 1. The op amp is in the active region (not saturated) 2. There is negative feedback 1k out out out out 1N914 (a) (b) (c) (d) 1k 1N914 out out (e) (f) (Student Manual for The Art of Electronics, Hayes and Horowitz, 2 nd Ed.)

23 Op-Amp Imperfections Linear range of operation: Input and output impedances not ideal values; gain and bandwidth limitations (including gain-bandwidth product); Nonlinear limitations: Output voltage swing (limited by rails and more); output current limits; slew-rate limited (how fast can the output voltage change: 0.5V/ms for the 741 op-amp, to 6000V/ms for high slew-rate op-amp); full-power bandwidth DC imperfections: Offset current (input currents don t sum exactly to zero); offset voltage; There are pins (denoted offsets) for inputs to help control this.