State University of New York at Stony Brook
Department of Electrical and Computer Engineering
ESE 211 Electronics Laboratory A
2011
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Lab 7: Operational Amplifiers
1. Objectives
1) Introduction to circuits based on Operational Amplifiers (Op-Amps).
2) Operation of Op-Amp in open loop configuration and with negative-feedback.
2. Introduction
Operational amplifiers are realized in the form of integrated circuits that have differential input (non-inverting
input V+ and inverting input V-) and single-ended output, VOUT. General-purpose Op-Amps have very large
differential mode voltage gain (ADM) and very large suppression of the common-mode voltage. In order to
perform quick circuit analysis it is often sufficient to use an idealized model of Op-Amp, i.e. assume infinite
differential-mode gain, zero common-mode gain, zero input currents (infinite input impedances) and zero
output impedance. For the circuit analysis in this lab we will use this model.
Typically Op-Amps utilize a bipolar power supply so they can operate with both positive and negative signals
without extra bias and coupling capacitors. The output voltage swing in op-amps is limited by the DC supply
voltages, VCC and VEE. In these experiments we use VCC = +10 V and VEE = -10 V, therefore the Op-Amp output
voltage cannot exceed ±10 V.
Op-amp circuit with no feedback.
In the Op-Amp without feedback (open loop) the range of input signal corresponding to linear amplification
regime is negligibly small due to the large differential mode gain. Hence in the open loop Op-Amp the output
voltage saturates at the values close to the power supply rails, VCC or VEE, depending on the sign of the input
voltage difference, V+ - V-. Thus, the op-amp with no feedback performs a comparison of two input voltages.
VOUT ≈ VCC = +10 V for V+ > V- ;
VOUT ≈ VEE = -10 V for V+ < VThe open loop configuration finds use in Analog-to-Digital converters and pulse width modulators.
The circuit shown in Figure 1 performs conversion of the slowly changing (almost DC) input signal into a
square-wave output voltage with the constant frequency and variable pulse width (duty cycle). This slowly
changing input voltage is compared with the saw-wave reference signal. For normal operation the range of input
voltage should be within the reference signal swing, |VIN| < |VREF|. For zero DC input voltage the output squarewave signal would have 50 % duty cycle.
Figure 1
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State University of New York at Stony Brook
Department of Electrical and Computer Engineering
ESE 211 Electronics Laboratory A
2011
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Op-amp circuits with Negative feedback.
Originally op-amps were designed for operation with analog signals in control systems utilizing feedback.
Figure 2 shows one version of the Op-Amp circuit with a negative feedback.
Figure 2.
Suppose a small positive signal VS is applied to node A. This signal creates a positive voltage difference
between inputs V- and V+. The Op-Amp quickly responds to this difference with large negative output voltage.
The output voltage with the help of feedback resistor RF brings the voltage difference between the inputs back
to zero. For small source signals the Op-Amp changes VOUT synchronously with VS to maintain the input voltage
difference at zero.
The circuit transfer function (voltage gain) can be analyzed with the first rule: V- = V+. It states that the voltage
difference between the Op-Amp inputs equals zero. The second rule assumes that no current can flow into the
Op-Amp input terminals. The input impedance of the Op-Amp with no feedback is assumed to be infinite,
because the Op-Amp inputs currents are zero at the finite input voltage.
Circuit in Figure 2 is so called inverting amplifier with voltage gain VOUT / VS = - RF / R1.
In contrast, in the Op-Amp circuit with negative feedback the equivalent input impedance between V- and
ground is zero because the Op-Amp input voltage is zero. Node V- (point B in Figure 2) is called a virtual
ground. If the source amplitude is too large, i.e. the output voltage saturates at the ±10 V levels, then the node B
cannot be considered as the virtual ground anymore.
If one applies signal VS to the non-inverting input, V+ (Figure 3), the Op-Amp output changes to maintain
voltage at V- the same as VS in accordance with the first rule. The output voltage is proportional to input signal
VS. Circuit in Figure 3 is so called noninverting amplifier with voltage gain VOUT / VS = 1 + RF / R1.
Figure 3.
Thus, the op-amp with negative feedback turns into a linear amplifier with the transfer function defined by
external impedances R1 and RF. The large value of differential gain is traded for a more accurate transfer
function and a wider frequency bandwidth.
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State University of New York at Stony Brook
Department of Electrical and Computer Engineering
ESE 211 Electronics Laboratory A
2011
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3. Preliminary lab
A. Consider circuit in Figure 1. Assume that DC is applied to inverting input and saw-wave reference is
applied to noninverting one. How the duty cycle of the output signal would change if value of DC is
increased? Answer the same question for DC applied to noninverting and saw-wave applied to inverting
input. Illustrate the circuit operation with the timing diagrams. Sketch the chip layout and show the
wiring connections.
B. Using the two rules formulated in introduction, derive the expressions for the voltage gains for the
inverting (Figures 2) and noninverting (Figure 3) amplifiers.
C. Simulate using PSPICE the frequency response of the open loop differential gain of uA741 Op-Amp in
the frequency range from 1 Hz to 1 MHz. Find the gain-bandwidth product. What would be the
frequency bandwidth of the 741-based amplifier shown in Figure 2 with R1 = 1 kΩ, RF = 20 kΩ?
4. Experiment
The experiments will be performed with an uA741 Op-Amp with the chip pin-out shown below:
1. Assemble the circuits from part 1 of the prelab. Apply a saw-wave with the amplitude of 1 V and the
frequency of 100 Hz. Measure and plot the duty cycle in the input voltage range from -1 to +1 V DC. Find the
coefficient between the duty cycle of the output square-wave signal and the input DC voltage.
2. Assemble the circuit in Figure 2 for the voltage gain of -5. Do not use resistors with the values below 1 kΩ
and larger 100 kΩ. Find the phase difference between the input and output voltages. Measure the actual voltage
gain in the frequency range from 10 Hz to 1 MHz using the sine-wave signal with the amplitude of 200 mV.
Determine the gain-bandwidth product from the measurements.
3. Increase the input signal amplitude to 4 V. Sketch the signal waveforms at nodes A, B and C. Explain the
nature of the signal change at node B.
4. Repeat 2 for Figure 3 circuit.
Report
The report should include the lab goals, short description of the work, the experimental data presented in tables
and plots, the data analysis and comparison followed by conclusions. Please follow the steps in the experimental
part and clearly present all the results of measurements.
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