When an op-amp circuit is being designed to operate at the edge of its gain capabilities, how can you determine whether a test circuit is stable? An op-amp design that is operating near the edge of its flat gain could exhibit oscillations, either as ringing, sustained oscillations, or growing oscillations. These behaviors are undesired as an amplifier should reliably produce a signal with the desired gain at the intended input frequency.
To make things more difficult, op-amp circuits designed to operate at high frequencies might pass evaluation in simulation, but the PCB layout could add parasitics that drive an op-amp into oscillation. Sometimes, op-amp oscillations may appear as desired behavior, when in fact they are problematic if the circuit is not designed correctly. Fortunately, it does not take much equipment to spot oscillations in the lab and determine steps to suppress them.
Op-Amp Oscillation Measurements
Op-amp oscillations can be evaluated in two ways: either by measuring an oscillation directly in the time/frequency domain, or by identifying the conditions that would produce an oscillation in terms of loop gain/phase. To take these measurements, only a few instruments are needed:
- Bench power supply (possibly more than one)
- Arbitrary waveform generator
- Oscilloscope and/or gain-phase analyzer
- Appropriate probes for the above instruments
If you’re probing an evaluation board for oscillations or instabilities, you likely not get the full picture of potential instabilities in real systems, which are affected by parasitics in the PCB layout and capacitance at the input/output/load. Instead, test fixtures should be applied to a test board to get accurate measurements that truly reflect the operation of the op-amp circuit.
The simplest measurement that can be used to identify oscillations is an oscilloscope. An arbitrary waveform generator is used to provide an input stimulus, either as a step input, ramped waveform, sine wave, or an arbitrary waveform. Typically a step response is used to examine the transient response, while a sine wave is used to examine the harmonic response. Typically the sine wave can be adjusted and the output examined to see when a significant oscillation arises.
When an op-amp circuit is stable, a sine wave input applied to an op-amp circuit will produce an output with the same frequency. If an input sine wave is causing the output to oscillate, the output will have a frequency that is different from the input. Some oscilloscopes will have an automated feature that can measure peak-to-peak frequencies and voltages, so you will be able to see when a driven instability arises on-screen.
This 3 GHz input signal (top graph) has excited a 3.7 GHz oscillation with extremely high total gain of 90 dB.
The other option is to look at a real-time FFT on an oscilloscope for both the input signal and the output. If an oscillation is excited, it will be visible as a high-Q peak with frequency that does not match the input. If sourcing with a square wave or a step function, a single peak corresponding to the oscillation frequency will be visible on top of the broadband response.
A more specialized measurement for determining the potential for oscillation in an op-amp circuit is a gain-phase analyzer. These instruments can take a range of measurements that are relevant for systems with feedback and control loops, including:
- Bode plots
- Nyquist plots
- Closed-loop and open-loop gain plots
While you could technically perform a loop gain measurement with a waveform generator and an oscilloscope, it requires manually tuning the sine wave frequency and measuring the gain/phase of the output. A gain-phase analyzer can automate this and will provide complete information about the op-amp circuit being investigated.
Loop gain/phase data will reveal the potential for an oscillation in an op-amp circuit when the loop gain = 0 dB and phase = -180° (or 0° phase margin). As the instability is approached, the gain-phase plot can start to appear very noisy because the phase margin starts approaching the instability point.
The expected instability point is marked at the on-screen cursor.
In general, there will always be some slight amount of oscillation as ringing, but we always prefer to keep the phase margin in the system greater than 45°. This condition can be verified by looking at the gain-phase plot.
Compare With Simulation
Time-domain responses are easy to examine in simulation with a transient analysis workflow. Op-amp models are commonly released from semiconductor manufacturers and these can be used in simulated op-amp circuits. It is possible to simulate the above measurements with your circuit design to identify instabilities in op-amp circuits. It is also possible to sweep through capacitances at different points in an op-amp circuit to test any compensation methods that target instabilities.
When you’re ready to simulate and measure your op-amp circuits and solve problems with oscillations, you can design and simulate your circuits with the simulation tools in PSpice from Cadence. PSpice users can access a powerful SPICE simulator as well as specialty design capabilities like model creation, graphing and analysis tools, and much more.