PY 452 Advanced Physics Laboratory Hans Hallen Electronics Labs

One electronics project should be completed. If you do not know what an op-amp is, do either lab 2 or lab 3. A written report is due, and should include a discussion of the circuit with inserted equations and plots as appropriate to substantiate the words and show that the measurements were taken. The report should not consist of the numbered items below and short answers, but should be one coherent discussion that addresses most of the issues brought out in the numbered items below. This is not an electronics course. Familiarity with electronics concepts such as input and , noise sources and where they matter, etc. are important in the lab AND SHOULD APPEAR IN THE REPORT. The objective of this exercise is to gain familiarity with these concepts.

Electronics Lab #1 Current Source

Task: Build and test a transistor current source, , +V with your choice of current (but don't exceed the transistor, , or R ratings). R1 Rload You may wish to try more than one current.

(1) Explain your choice of R1, R2, and Rsense. VCE (a) In particular, use the 0.6 V drop of VBE - rule for current flow to calculate I in R2 VBE Rsense terms of Rsense, and the Ic/Ib rule to estimate Ib hence the values of R1&R2.

(2) Try your source over a wide range of Rload values, plot Iload vs. Rload and interpret. You may find it instructive to look at VBE and VCE with a floating (neither side grounded) multimeter.

(3) What is the output impedance of the current source? What loads () can it drive? Measure it.

(4) Think about another current source: 100 k ohms 1 kV (11 90V batteries Rload so won't get power supply ripple) , • Compare this to your transistor current source (plot them together -- use theory for this one). • Which is better: for current regulation? for noise? (think about all sources) • This is a "physicist's source" of the brute force approach -- not something you would sell, but could help in a low noise need-it-now type of environment.

1

Electronics Lab #2 Op-amp Differential Task: Build and study a simple operational amplifier-based , R1 R3 V- - Vout + V+ R2 R4 . Use a gain of 1 (R1=R2=R3=R4= 10k or 100k ohm is a good impedance) (1) DC properties (use ElectronicsLab.vi) (a) Derive the output in terms of the common mode voltage Vcm, (V+ + V-)/2), and differential voltage Vdiff, (V+ - V-), using the golden rules. Hint: derive first in terms of V+ and V-. then change variables. (b) Choose several values of V+ and V- (think before taking data), and plot the output voltage as a function of the Vcm or Vdiff to find the gains (cm and diff). Compare to the calculations. (c) The common mode rejection ratio is (CMRR) = (differential gain) / (common mode gain). Compare your measurements to the golden rule prediction of the CMRR. (d) Check the effects of R1 ≠ R2 by replacing R2 with a different value . Repeat (b-c) and compare them to your calculations in (a) with the current R2. (2) Impedances: Think of your entire circuit (not just the op-amp) as a black box, what does it look like from the outside? Input: model as a resistor, measure by V/I, set one, measure other. Output: think of as a perfect in series with a resistor, measure it as noted below. (a) What is the input impedance of the amplifier? Do you worry about the "output impedance" of the driving sources? (look at your answer to 1(d)) How can you insure matching driving impedances? (b) What is the output impedance of the amplifier? (Measure V with different load R’s) Use the resistor substitution box. Plot Iout vs Rload, note when the op-amp current limits, and ignore those data when calculating output impedance. (3) Frequency response: Add a capacitor as shown so that 1/(2πRC) = 1 kHz. Calculate with R1=R2=R3=R4 and C || R as one of the Z's that replaces R3 and R4 in the 1(a) calculations. C R3 V vs. time sine R1 - osc. trace 1 + R2 trace 2 R4 C

. (a) Plot the magnitude (log-log axes) and phase (semilog axes) as a function of frequency. Acquire both the oscillator and output to estimate phase (use ElectronicsLabGainPhase.vi). Plot data as dots and theory as lines, recall capacitor has +/- ~20% tolerance. (4) Noise: Determine the major sources of noise in the amplifier. Describe the sensitivity to pick-up from various noise sources.

2

Electronics Lab #3 Current Preamplifier

Task: Build and study a simple operational amplifier-based current pre-amplifier, Rfb

Iin - + Vout .

(1) Use the Golden Rules for op-amps to calculate the Vout/Iin. (a) Choose Rfb for microamp or smaller currents. (b) Design a (voltage source/resistor) current source for testing.

(2) Verify the behavior experimentally. (a) First the DC properties (you can use ElectronicsLab.vi with a suitable 1(b) result). (b) Insight: use a triangle wave source and couple it capacitively (wrap the wire from the signal generator around the input wire. Do not touch metal to metal.) Can you explain the output wave-form? (hint: q = CV)

(3) Modify the calculations for an op-amp of gain G: Vout = G(V+ - V-). (a) What is the input impedance of the preamplifier? (b) What should the input impedance be? (c) Do you think you can measure it with G ~ 10^6? (Try to in your circuit) (d) What is the output impedance of the amplifier? (Measure V with different load R’s) Use the resistor substitution box. Plot Iout vs Rload, note when the op-amp current limits, and ignore those data when calculating output impedance.

(4) Frequency response: add a capacitor as shown so that 1/(2πRC) = 10 kHz. Incorporate Z from Rfb || C into the calculations of (1) for modeling here. C V vs. time Isource Rfb sine R - osc. trace 1 + trace 2

. (a) Plot the magnitude (log) and phase (lin) as a function of frequency (log scale for both). Acquire both the oscillator and output to estimate phase (use ElectronicsLabGainPhase.vi). Plot data as dots and theory as lines, recall capacitor has +/- ~20% tolerance.

(5) Noise: Determine the major sources of noise in the amplifier. Describe the sensitivity to pick-up from various noise sources.

3

Electronics Lab #4 Power Supply

Task: Build and study a simple unregulated power supply, then regulate it. full wave rectifier Filter Capacitor Unregulated 12.6 V Output transformer box Rload . Start with a load of ~100 mA, and watch the direction of the !!!! (1) Build up the circuit shown one component at a time from the left (always with Rload). (a) Look at/sketch the waveform at each stage. You can grab them with two channels of (differential input) data acquisition and compare them as plots. (b) Can you explain each waveform?

(2) Study the effect of the value of C on the residual ripple. (a) Use 1,2,3,... capacitors in parallel noting the difference. (b) make sense of the results in terms of Q = CV (or I = C dV/dT). (c) Vary the load.

(3) Regulate the power supply: A 9V battery works well fofr Vref. The transistor must handle the , current, and power. Use a top-hat (TO-3) transistor from the shelf. full wave rectifier Filter Capacitor Regulating transistor Regulated 12.6 V Output transformer box Vref Rload

(a) Does the output voltage compare to Vref as expected for a transistor (Ib hence Ic flows for Vbe > .6V and Ic/Ib is a big number). (b) How close to the Input voltage can you regulate. (c) Vary the load -- does the transistor get hot? Can you think of protection circuits that would keep it safe?

(4) What is the output impedance of the power supply (what should it be if it were perfect)? Measure your's. What range of input impedances can it drive? Do they change with frequency (thinking question)?

(5) Noise: Determine the major sources of noise in the voltage output. Describe the sensitivity to pick-up from various noise sources.

4