Gain vs. Bandwidth and Current Feedback Operational Amplifiers
In this lab we will demonstrate the inverse relationship between closed-loop
gain and bandwidth in conventional operational amplifiers, then experiment with
a different type of operational amplifier whose gain and bandwidth are
independent of each other (the current feedback amplifier). The amplifier
which we will use is the Burr Brown Model OPA658, which is designed for low gain
(1 to 5 inverting or non-inverting), wide bandwidth (200+ MHz) applications.
In working with high-speed op amps, it is necessary that your circuit be
neatly constructed to minimize stray capacitances, that a single, short ground
bus be used, and that the supply voltages be bypassed at the op amp.
A. Conventional Op Amps
- Construct a non-inverting amplifier using a 411 op amp and measure its
small-signal 3 dB bandwidth for closed-loop gain of 1.
signal, we mean that the output amplitude is sufficiently small that the slew
rate of the op amp is not a limiting factor at the 3 dB frequency.)
Increas the gain to 10 and measure the bandwidth.
Increas the gain to 100 and measure the bandwidth.
the feedback ratio beta and expected value of the bandwidth in each case?
- A unity-gain inverting amplifier has half the bandwidth of a unity
gain non-inverting amplifier (a follower). This is because the feedback
ratio of the inverting amplifier is the same as that of a non-inverting
amplifier of gain 2.
Measure the bandwidth of a non-inverting x2 amplifier
Measure the bandwidth an inverting x1 amplifier and see if they agree, and compare with the
bandwidth of the follower amplifier from the first part.
B. Current Feedback Op Amps
NOTE: The supply voltages for the OPA658 op amps CANNOT exceed +- 5.5 volts, and
typically operate off of +- 5 volts. Adjust your breadboard voltage
to +- 5 or slightly lower
volts before going further.
- Design and breadboard an inverting amplifier having a gain of -1, using the
recommended value of the feedback resistor R2.
Check the small signal
operation using a sinusoidal input, and test the frequency response of the
amplifier. Are you able to measure the 3 dB frequency?
- Then measure the small
signal step response. Is the step response limited by the amplifier or by the
risetime of your function generator or oscilloscope?
- What bandwidth would the step response imply? Compare with the op amp
- Measure the saturation levels.
- Measure the slew rate of the amplifier for a large output signal and
compare with specs. Make sure it is not saturating.
- The open loop specs of the amplifier indicate that one can decrease
the bandwidth of the amplifier by increasing the size of the feedback
Use this to design an amplifier having a 3 dB bandwidth of 5 MHz
and an inverting gain of 2.
- Measure the gain and bandwidth and compare.
the gain to 20 by reducing R1. Does the bandwidth change? Measure and Explain.
© Copyright 2003 New Mexico Institute of Mining and Technology