Exercise 3 Operational Amplifiers and feedback circuits

Transcription

1 LAB EXERCISE 3 Page 1 of 19 Exercise 3 Operational Amplifiers and feedback circuits 1. Introduction Goal of the exercise The goals of this exercise are: Analyze the behavior of Op Amp circuits with feedback. Measure the parameters of amplifiers built with Op. Amp. Verify differences from the behavior expected using ideal Op Amp models. As in every exercise, a comparison between calculations, simulation results, and the measurements is required. In this experience, some of the experimentally verified behaviors are aimed to point up the limits of simplified models used during the lessons Circuits and instruments to use The required circuits are preassembled; during the exercise you just have to connect the instruments to the measurement points (power supply, signal generator, oscilloscope and multimeter). Only the A3 module (Amplificatori) is used in this experiment. Notes: Some measurements require to add DC component to the signal; use the offset command on signal generator. The prewired boards require power supply; use 12 V and 10 V. Review the instructions on Power Supply Units in exercise 2. Here the instructions specify asymmetric power supply voltages, which in some experiments must be modified independently from each other. Therefore it is not possible to use the tracking mode. Any report about experimental measurements must show information enabling to rebuild the conditions in which measurements were accomplished. Remember to specify the characteristics of the instruments used (brand, type, serial number), and to check (at least qualitatively) that the instruments are working and that data on their nameplate match with what is required for the experience. Setup of Breadboard The circuits for the measurements are preassembled on printed circuit boards designed for these exercises; the circuit configuration can be modified using switches. Use the coaxial connectors (BNC) for input signals, the 3pin connector for power supply, and the test points to connect the oscilloscope probes or other instruments. The connection of external instruments is similar to the previous experiment. There is no need to insert or change any external component to complete this exercise.

2 LAB EXERCISE 3 Page 2 of 19 Homework In some occasions, you will be requested to compare your measurements with the results of calculations and/or simulations. Calculations and simulations must be carried out before the lab experience, using numeric data you can find in the lab guide. Numeric data consist of components nominal value and tolerances; calculation and simulation can be carried out either using only the nominal values or (better) using the range of possible values according to tolerances. Also result of the measurements are affected by imprecision of instruments and other errors. Thus, one should expect a mismatch between calculations/simulations and measurements (on the contrary, identical values should induce some doubts about the correct execution of measurements). The range of values due to components tolerances and the one due to errors of measurement must have overlapping areas. In these experiences you are not expressly required to make a quantitative verification of this correspondence; however it is useful to express some brief qualitative considerations. For each measurement you have to use one of the preassembled circuits, prepared according to the configuration shown in this manual. Note: In this first experience you have to use active circuits, requiring power supply. Before connecting power supply, set the output voltages to the correct value. The instruments inside the power supply unit are generally not accurate, thus it could be convenient to verify voltages with an external instrument (tester or digital multimeter). When you use a signal generator you must always ask yourself which output level it has to be set to. If signal voltage is too low, carrying out the measurements could be difficult, while too high values could damage some components, or bring the circuit away from the assumed working conditions (e.g. out of linear operating area). The results of the measurements must be written in tables already prepared, which you can find in the draft report at the end of this document. You have to deliver the report completed with the required data (only pages with measurement data, not the full instruction booklet).

3 LAB EXERCISE 3 Page 3 of and simulations 2.1 Parameters of a voltage amplifier Module setup Use the board A31 and configure it according to the following stepbystep instructions. In the following the term Amplifier indicates the complete circuit (schematic inside the box). S3 J3 V S R3 J7 V I J4 ua741 R2 12k R1 100k J8 J2 V U S7 R5 2,2k Switch table S1 S2 S3 S4 S5 S6 S7 2 closet 1 R3=4.7k inserted 2 R3 shortcircuited 2 closet 1 R5 not inserted 2 R5=2.2k inserted Homework Evaluate the amplifier gain. Estimate (order of magnitude) the input and output equivalent resistances, for the following Op Amp parameters: Rid = 1MΩ; Ro = 100Ω; Ad = Simulate the circuit using the Op Amp model provided in Pspice, and evaluate gain and equivalent input and output resistance.

4 LAB EXERCISE 3 Page 4 of 19 Measure the gain Vu/Vi. (set Vs = 0,5Vp sine, signal frequency f = 2kHz ). Use the scope to verify circuit operation and for measurements. If available use a DVM to measure signal amplitude (ACV). Warning: the gain is Vu/Vi, not Vu/Vs. Operating on S3 and S7, verify that input and output resistances are respectively very high and very low (qualitative only; measurement in the next point). Measuring the equivalent input resistance A suitable technique for measuring the input resistance of any module is to put a resistance between the signal generator and module input. The new resistance and the input resistance are a voltage divider, where the unknown element can be evaluated from measurements of signals at input and output of the voltage divider. This is the same technique used in the previous lab exercise, and can be used for any electronic module. For more precise measurements: use additional resistance of the same order of magnitude of the (estimated) input resistance. To avoid perturbation to the circuit under measurement (such as measuring the instrument resistance, when input resistance is very high), measure the output voltage and insert/remove the additional resistor. The actual significant figure is the ratio of voltages, which does not change (as long as the amplifier operates in linear zone). For the circuit used in the lab, S3 allows to insert/remove R3 (S3 closed shorts the resistance, and Vs is directly applied as Vi). Measure the output signal in the two cases, and get the input resistance from the ratio (knowing the amplifier gain is not necessary). Compare measurement, simulation, and homework results. Measuring the equivalent output resistance Equivalent output resistance Ro can be measured as Ro = (Voutnoload)/(Ioutshortcircuit) only when solving an exercise on the paper. In most cases, a short circuit at the output of an amplifier modifies radically the working conditions, and drives the circuit out of linear region. The usual approach for lab measurement is similar to the technique used for input resistance: create a voltage divider with one known element, and measure two voltages. At the output we can connect a load (not so heavy to drive the circuit out of linearity this condition can be verified with a scope), then measure the signal change from noload to knownload, and apply the voltage divider equation. Compare measurement, simulation, and homework results.

5 LAB EXERCISE 3 Page 5 of Parameters of an inverting voltage amplifier Module setup Use the board A32 and configure it according to the following instructions. Switch configuration S8 S9 S10 S11 S12 S13 S14 2 closed 1 R11 not inserted 1 R12 not inserted J9 V I J14 R9 22k J10 J12 R10 100k ua741 R8 10k J11 V U J13 Homework Evaluate the amplifier gain. Estimate (order of magnitude) the input and output equivalent resistances, for the following Op Amp parameters: Rid = 1MΩ; Ro = 100Ω; Ad = Simulate the circuit using the Op Amp model provided in Pspice, for the configurations mentioned in a) b) c) d) of the next Measurement section. For the following measurements apply at input a triangular wave with amplitude 2 Vpp, DC = 0, period = 3 ms. a) Evaluate the gain measuring the signal at input and at output b) Verify the voltage level on the noninverting input () of the Op Amp (should be close to 0). Use scope or DVM. c) Verify the DC and AC voltage on the inverting input (both should be close to 0). d) Increase input signal amplitude till about 5 Vpp; the output should exhibit high distortion (clipping, saturation). e) Verify the voltage on the inverting input of the Op Amp (differential input voltage) when the output shows distortion. f) Verify the effects of supply voltage on distortion and on the differential input voltage (modify each supply voltage up to 2V).

6 LAB EXERCISE 3 Page 6 of Differential amplifier Module setup Use the board A32 and configure it according to the following instructions. In this circuit the switches S8 S11 allow to get as V2 (on J1) a voltage corresponding to a known fraction of V1, through the voltage divider made by R6 R8. Close one switch at a time in the group.s11, leaving all the others open. The two voltages Vi and V2 allow to verify the operation of a differential amplifier with a single signal generator. Switch configuration J9 J10 R10 100k S8 Select V2 = V1 S9 Select V2 = 2/3 V1 S10 Select V2 = 1/3 V1 S11 Select V2 = 0 S12 2 closed S13 1 R11 not inserted S14 1 R12 not inserted V I R6 10k R7 10k R8 10k J14 S8 S9 S10 S11 R9 22k V 2 J12 ua741 J11 V U J13 Homework: Evaluate Vu(Vi) for the various configurations of SW Apply at input a sine signal, 0,5 V eff, f = 200 Hz. Measure the gain A V = V U /V I for the various configurations (use the scope or the DVM). Compare measurement and homework results.

7 LAB EXERCISE 3 Page 7 of AC amplifier Module setup Use the board A32 and configure it according to the following instructions. The switches allow to set the module as Dc or AC amplifier, with various gain and bandwidth. J3 S4 C5 100 nf V S V I J7 J5 R4 10k LM358 R1 100k J6 S6 R2 12k S1 C3 150pF 10nF J8 J2 V U S2 C4 100 nf Switch table S1 S2 S3 S4 S5 S6 1 C3 (10nF) not inserted 2 C3 inserted 1 C4 (100 nf) inserted 2 C4 not inserted (short circuited) 2 closed 1 C5 (10 nf) inserted 2 C5 not inserted (short circuited) 2 closed Homework Evaluate the effects of operations e), f), g) described in the next section ().

8 LAB EXERCISE 3 Page 8 of 19 Set the circuit as DC amplifier: S4 closed, S2 closed, S1 open, a) Measure the gain at 100, 1k, 10k, 100k Hz (1); b) Find the frequencies where the amplifier gain has a 3 db drop (that is the upper band limit, corresponding to the HF pole position). Use a low level input signal, to avoid distortion at output. To carry out this measurement, start with signals in passband (max gain); set the amplitude to get a full signal excursion (vertical) over the scope screen. Then increase the signal frequency till the value where the output response drops 3 db (0,707). c) Apply a squarewave signal, with suitable amplitude to keep the circuit in linear operation. Since a squarewave does not allow to see directly saturation, verify linear operation by applying small changes to the input signal amplitude, and checking the changes at the output. Set scope time scale and signal to see the exponential shape of the rising and falling edges. With small signal you can see exponential edges; with higher signal levels the edge becomes linear, due to dynamic saturation of the amplifier (Slew Rate limit). Select the squarewave frequency and the scope time scale to see the lowpass and the highpass behavior. For lowpass use high frequency squarewave and fast horizontal sweep; for highpass use low frequency squarewave and slow sweep (2). d) Apply an offset from the signal generator, and verify the effect at the output (offset amplification). e) Insert C3 (close S1; keep S4 and S2 closed); the following points do not need precise measurements, only qualitative check: e1) verify the change in transient response (squarewave) e2) verify the effect of C3 on high cutoff frequency (using sine signals) e3) verify the DC gain (change the signal generator offset). The output DC comes from several sources: DC component of input signal, offset, power supply unbalance. To measure the actual DC gain apply DC change at the input (signal generator offset), and neasure the effect at the output, and evaluate the ratio of changes. f) Insert C4 (open S2; keep S4 closed and S1 open) f1) verifythe effect on frequency response f2) verify the effect on transient response. g) Insert C5 (open S4; keep S2 closed and S1 open): g1) verify the effect on transient response g2) Verify the effect on DC gain (change signal generator offset)

9 LAB EXERCISE 3 Page 9 of 19 h) Remove C5 (S4 closed) and leave other switches in the last position. Apply a lowamplitude squarewave (keep the amplifier in linear operation). Set the frequency to see an output a signal as in the figure. From measurements on the waveform evaluate the time constants associated with capacitors C3 and C4 (keep C5 shortcircuited; use two different time scales for measurements). Vu t The two examples show the same signal with different setting for sweep speed (time scale) on the scope. Vu Vu t t Lowpass behavior, high speed sweep Highpass behaviour, low speed sweep Remarks: 1) In the measurements at high frequency, that is with high scan speed on the scope, besides the bandwidth limit of the Op Amp, you see also the effects of the limited Slew Rate. The effect of slew rate saturation is to change sine signal into triangular waves, with slope corresponding to maximum slew rate. To measure the bandwidth or the transient response of the amplifier verify the output wavefor. If the signal becomes a triangular wave, reduce the input signal level, to keep a sine output. In the transient response the slew rate saturation causes linear rising and falling edge (instead of exponential). 2) For the measurement in point c), sometimes the response shows also II order behavior, such as damped ringing. This effect is caused by parasitic L and C elements in the wiring, and is not related with the behavior analyzed in this lab experience. With slow time base on the scope, the ringing is embedded in the signal edges and is not visible.

10 LAB EXERCISE 3 Page 10 of 19 Complete schematic of the measurement board A3. Module A3 1. J3 S3 V R S 3 4,7k J7 J4 C nf S4 R 4 10k S5 J5 LM358 R 1 100k R 2 J2 S1 S6 150 pf Vu C3 J8 S7 R 5 2,2k 12 k J6 S2 C 4 100nF Module A3 2. J9 J10 R 9 22 k R k J11 J14 V S S12 J12 S8 R 6 10 k S9 R 7 10 k S10 LM358 V 2 J13 S13 Vu R11 1 k S14 R W R 8 10 k S11

11 LAB EXERCISE 3 Page 11 of Report form (draft) Exercise 3: Operational Amplifiers and feedback circuits Date: Team ; composition: role name signature Report writing Instruments used Instrument Brand and type characteristics Signal generator: Oscilloscope Power supply Preassembled circuit Brief description of goals