Crossover distortion?

Hello Claire,

Every op amp, including the power op amps, has an open-loop gain/phase vs frequency response similar to that shown below for the OPA541. The actual gain and phase at a frequency will be unique for each different op amp because of the different designs and processes; however, the OPA541 curves are useful for illustrative purposes.

The OPA541 open-loop gain (Aol) at very frequencies is about 100 dB. If the op amp is used in this frequency range and a moderate to low closed-loop gain (Acl) is needed, there is high loop-gain available to assure high gain accuracy and to reduce output distortion. However, as the frequency of interest is increased the Aol decreases at a -20 dB/dec rate and falls to 0 dB around 2 MHz. Thus, the Aol decreases at higher frequencies leaving less loop-gain available to reduce output distortion.

Your description of the OPA544 "hiccup" and image of the LM675 output appear to be output crossover distortion. This is normal behavior for a class AB output stage used in most op amps. This kind of distortion is much reduced when the loop-gain is very high at the lower frequencies, but increases as the Aol decreases.

If the maximum frequency can be kept in the 20 to 50 kHz range, then reasonably good performance may be attained with the OPA541, OPA544 or LM675. The slew rate would also have to be considered to make sure it supports the needed output voltage swing without introducing slew rate limiting. Do note that the LM675 is designed to be used in applications where the Acl is 10 V/V, or greater. Anything lower then instability is likely to occur.

Applications where high output current capability and low distortion are required at higher frequencies often employ a wideband RF power amplifier; one designed to deliver watts of output power at low distortion levels.

Regards, Thomas

Precision Amplifiers Applications Engineering

Hi all,

Thank you for that helpful information. Attached is the customer schematic provided. Does this help give you an idea as to the instability?

Additional information:

Above is a drawing of a section of the circuit which currently uses the LM675T. The op amp is attached to heat sink and dead bugged into the proper nodes on a vector board. Vsig is a sinusoid which will go as high as +/-0.5V and up to 1Mhz. If Voffset = 0, the crossover “hiccup” occurs, the hiccup is about 2 us. As the frequency is increased, it becomes more noticeable and the sinusoid becomes “less pure”. Frequency does not seem to alter the 2us crossover time. If DC offset (positive or negative) is added enough to where the AC signal doesn’t cross 0, the sinusoid looks great, no visible difference between the signal and the LM685T output. If increased up to 1.5-2A with a DC offset so there is no 0 crossing, and up to 200kHz and there isn’t any noticeable distortion in the output. Looking for an op amp, which has a minimal crossover time so up to 20Khz (more comfortable with 50-200kHZ max) can be achieved, < 1% purity error in the sin wave output.

Interesting instability, if a DMM is wired in series with the 1 ohm resistor load to measure the current, the LM675T seems to go nuts and gives a 2A DC output and gradually decreases, it also shows large amounts of noise added to the Vsig +in line. This behavior is the same whether the cap and resistor connector -in or +in are present or not. This behavior did not happen when OPA544 was used. This seems to be repeatable. The OPA544 does have about a 10us crossover hiccup though.