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 National Semiconductor LM6181, which is designed for low gain (1 to 10 inverting or non-inverting), wide bandwidth (100 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. (By small 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.)
• Increase the gain to 10 and measure the bandwidth.
• Increase the gain to 100 and measure the bandwidth.
• What is 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 LM6181 op amps CANNOT exceed +- 18 volts, and typically operate off of +- 15 volts. Adjust your breadboard voltage to +- 15 V. Be sure to have power bypass capacitors close to chip.

• 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 rise time of your function generator or oscilloscope?
• What bandwidth would the step response imply? Compare with the op amp specs.
• 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 resistor.
• 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.
• Increase the gain to 20 by reducing R1. Does the bandwidth change? Measure and Explain.