Op Amp Recovery and Slew Rates

When working with op amps, it’s generally a bad idea to drive the op amp output “rail to rail”. Why? Because it takes longer for the op amp output to recover from saturation and begin linearly tracking the input signal again. The actual recovery time of an op amp, and how badly this recovery time will affect your circuit, depends of course on the op amp and your circuit.

Let’s consider a simple full-wave rectifier circuit:

A simple full-wave rectifier

A simple full-wave rectifier

When the input signal is negative, the op amp drives its output positive, placing diode D1 into conduction, which results in one-half of the rectified signal at the output:

LM741 half wave rectification at 10Hz

LM741 half wave rectification at 10Hz

When the input signal is positive, the op amp drives its output negative, reverse-biasing D1. With D1 effectively an open circuit, the positive input signal arrives at the circuit’s output via resistor R2, providing the other half of the rectified signal:

LM741 full wave rectification at 10Hz

LM741 full wave rectification at 10Hz

The rectified signal certainly isn’t useful for applications that require precision, but if you only need a rough rectification, this works fine at lower frequencies.

At higher frequencies however (say, 10kHz), the waveform becomes severely distorted:

Rectifier output at 10kHz

Rectifier output at 10kHz

To better see what’s going on here, let’s take a look at the input waveform as well:

Rectifier output (blue) vs input (yellow)

Rectifier output (blue) vs input (yellow)

The output is tracking the input, but there’s a problem: the half of the rectified waveform that traverses resistor R2 keeps driving negative for 16us before the output suddenly shoots positive again. The reason becomes much clearer when we look at the voltage of the op amp’s output pin (connected to the anode of D1):

Voltage at D1's cathode (blue) vs the anode (yellow)

Voltage at D1’s cathode (blue) vs the anode (yellow)

It’s obvious that after being driven all the way to the negative rail, it takes some time for the op amp to make its output positive enough to turn D1 back on and begin conducting during the next half of the cycle.

Part of this is due to the fact that most op amps will take some time to recover from saturation. It is also due in part to the slew rate of the op amp, which for the LM741 is only .5V/uS.

A simple solution is to add a second diode, D2:

Addition of diode D2

Addition of diode D2

D2 ensures that the op amp never drives its output too negative; this avoids saturation recovery time, and partly makes up for the LM741’s low slew rate by limiting the distance its output has to travel in order to forward-bias diode D1:

LM741 output with the addition of diode D2

LM741 output with the addition of diode D2

The waveform certainly isn’t perfect, but its response time has been improved by about 4us which is a 25% improvement.

Another solution is to just use an op amp with a much faster slew rate. The TL081 has a slew rate of 13V/us and fares better than the LM741 even without the addition of diode D2:

TL081 output, without D2

TL081 output, without D2

In fact, the addition of diode D2 has little effect on the output of the TL081 in this case:

TL081 output, with D2

TL081 output, with D2

There are of course better op amps for this particular application, and better circuits to use for full-wave rectification, but this illustrates the importance of understanding saturation recovery times and slew rates, and their effects on your circuit performance.

Leave a Reply

Your email address will not be published. Required fields are marked *