Lab 10: Single Supply Amplifier

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1 Overview This lab assignment implements an inverting voltage amplifier circuit with a single power supply. The amplifier output contains a bias point which is removed by AC coupling the output signal. AC coupling is accomplished by placing a capacitor in the signal path; the capacitor acts as an open circuit to constant voltages, so that constant values are removed from the signal. Symbol Key: nstrate circuit operation to teaching assistant. ; include principle results of analysis in laboratory report. Record data in your lab notebook. General Discussion: It is often desirable to use only a single power supply to provide power to an operational amplifier. This implies that one of the operational amplifier rails will be grounded, which restricts the range of output voltages from the operational amplifier to either purely positive (if the negative power supply is grounded) or purely negative (if the positive power supply is grounded). This restriction on the range of output voltages can cause problems if, for example, the goal is to amplify a signal which contains both positive and negative values. Under these circumstances it is, of course, necessary for the output to contain both positive and negative components. One approach toward resolving this problem is to create a bias point in the output voltage. The bias point essentially re-defines where zero voltage is. The output signal then consists of the desired output superimposed over this bias voltage. The bias voltage can be removed from the output signal by AC coupling the signal this simply removes any constant components of the signal, including the bias point. I. Inverting Voltage Amplifier Figure 1 shows an inverting voltage amplifier with a single supply. The voltage at the positive power supply is V+, and the negative power supply is grounded. Our goal for this circuit is that the output voltage is an inverted and amplified version of the input voltage that is, the output waveform is the same shape as the input waveform, except that their amplitudes are scaled by a negative constant. 1

2 The voltages labeled on Figure 1 are all relative to ground. The assumed polarity on these voltages is that ground is the assumed negative voltage terminal and the labeled voltage is the positive terminal. These conventions are common on electronics diagrams. Figure 1. Single-supply inverting voltage amplifier. Pre-lab: a) Determine V o as a function of V i, R 1, and R 2 for the circuit of Figure 1, ignoring the limitations associated with the op-amp power supplies. b) Assuming that the positive op-amp supply voltage is higher than the highest value of V o and that the negative power supply is grounded, sketch what you would expect as the input and output voltage waveforms if V i(t) = Acos(ωt). Label the maximum and minimum values of both waveforms on your sketch in terms of A, R 1, and R 2. Lab Procedures: 1. Implement the circuit shown in Figure 1, using R 1=1kΩ, R 2=10kΩ, R L = 47kΩ, and V + = 5V. 2. Set the input voltage V i to be a 1kHz sinusoid with amplitude 100mV and no offset. Use your oscilloscope to measure the voltages V i and V O. Record an image of the oscilloscope window, showing the V i and V O waveforms. 2

3 Comment on the agreement between the measured waveforms and your expectations from part (b) of the pre-lab. 3. nstrate operation of your circuit to the Teaching Assistant. Have the TA initial your lab worksheet indicating that they have observed your circuit s operation. II. AC Coupling The circuit of Part I does not behave as desired. Since the negative voltage supply is grounded, output voltages resulting from positive input voltages are not scaled versions of the input voltage. Reminder: The supply voltages limit the range of possible output voltages. Any attempt to exceed this range causes the op-amp to saturate, and the equation governing the relationship between the input and output is no longer valid. In our case, this means that our circuit won t do what we want if the input is negative. The circuit of Figure 2 can be used to provide both positive and negative timevarying output voltages from a single-supply amplifier 1. In order to amplify the input signal V i and retain its overall shape without having access to negative values, we bias the circuit to some non-zero voltage. The two resistors, R B1 and R B2, and the secondary voltage V CC sets this bias point by applying a non-zero voltage to the noninverting op-amp input. Now the output from the operational amplifier consists of an amplified version of the input signal superimposed over a constant bias voltage. The input signal and output signal must, however, be isolated from this bias voltage in order for the circuit to behave as desired. The capacitors C 1 and C 2 will isolate the input and output from this bias voltage, since capacitors look like open circuits to DC voltages. When the circuit is actually implemented, we will use the same voltage supply to apply V + and V CC. This will retain the single supply characteristic of the circuit. 1 This is not true for constant voltage inputs. This will become apparent when you analyze the circuit in the pre-lab. Luckily, the desire to amplify time-varying signals is so common that the circuit of Figure 2 (and many similar circuits) show up in a lot of applications. 3

4 Figure 2. AC coupled single supply inverting voltage amplifier. Pre-lab: Superposition is used to analyze the circuit of Figure 2. The voltages and currents in the circuit will contain two separate components: a constant (or DC) value introduced by the bias point, and a sinusoidal input (or AC component) that we want to amplify. of the circuit thus consists of analyzing these components as if they were two separate inputs: (a) DC component: to determine the constant values in the circuit of Figure 2, replace the capacitors with open circuits. Determine the DC values of the voltages at V OA and V O. These values may be functions of the circuit parameters R 1, R 2, R B1, R B2, and the voltages V+ and V CC applied to the circuit. (b) AC component: the AC voltages at V OA and V O in the circuit of Figure 1 can be estimated by replacing the capacitors C 1 and C 2 with short circuits 2, and calculating the circuit s response. Determine the AC values of the voltages at V OA and V O. These may be functions of the circuit parameters R 1, R 2, R B1 and R B2, and the voltages V+ and V CC applied to the circuit. (c) Total response: Determine the waveforms at V OA and V O by superimposing the results of parts (a) and (b). Sketch the voltage waveforms at V OA and V O. Label the maximum, minimum, and average values of both waveforms on your sketch. (As in parts (a) and (b), these labels may be functions of the circuit parameters and supply voltage values.) 2 We are assuming that the frequency of the sinusoidal signal applied to the input is very high. 4

5 Lab Procedures: 1. Implement the circuit shown in Figure 2 below. Measure and record all resistor values. Use V CC = 5V, and set the input voltage V i to be a 1kHz sinusoid with amplitude 100mV and no offset. Use your oscilloscope to measure the voltages V i, V OA, and V O. Record an image of the oscilloscope window showing the V i, V OA, and V O waveforms. Set the oscilloscope measurements to provide at least the amplitude, AC value, average value, and frequency of these three waveforms. If you are using a 2 channel oscilloscope, record two oscilloscope window images; one showing V i,and V OA, and the other showing V i,and V O. Figure 2. Amplifier to be implemented. 2. Based on your pre-lab analysis and your measured values for the circuit resistances and capacitances, calculate the expected AC amplitudes of V OA and V O, and the DC (average) values of V OA and V O. 3. Calculate percent differences between the expected and measured values for the AC amplitudes of V OA and V O, and the DC (average) values of V OA and V O. 4. nstrate operation of your circuit to the Teaching Assistant. Have the TA initial your lab worksheet indicating that they have observed your circuit s operation. 5