Op-amps driving I'm not sure what they are called capacitors

Mag748

Hello,

I hear a lot that capacitive loads are bad for op-amp outputs. But can an op-amp drive multiple capacitively I'm not sure what they are called lines? Take a look at the attached schematic image. Will this cause problems?

Thanks,
Marcus

P.S. The capacitor on the left is 22uF, and the 4 on the right are 10uF.

Leon

This is nothing to do with decoupling! Decoupling capacitors are typically put across supply and ground pins.

Mag748

My apologies Leon. I will edit my post right now.

Leon wrote: »

This is nothing to do with decoupling! Decoupling capacitors are typically put across supply and ground pins.

ctwardell

Marcus,

In this case they are being used as coupling capacitors. The purpose is to block the DC bias voltage and pass the desired AC signal.

Do some google searching on "amplifier coupling capacitors".

C.W.

LoopyByteloose

Capacitors can be used in a variety of ways. Each requires understanding a different point of view.

1. In converstion of AC to DC power supplies, capacitors are used as a filter to reduce ripple. The goal is to eliminate a 50/60 cycle hum.
2. In oscillators, capactiors are often used with coils to tune to a particular frequency
3. Between amplifier stages, the capacitors can be used to block the DC portion of signal while letting the AC portion pass (with some limits on performance)
4. Also between amplifier stages, capacitors can be used to create a wide variety of passive filters.
5. Capacitors can be used in digital power distriubtion to block transients, another form of filtering.

You may be a bit confused about the differences between 3 and 4, but in most amplifier stages, the coupling capacitor is somewhat equavelent to a high pass filter where the blocked frequency is so low that it is not important.

The above only is mentioning passive filtering. Also, one can use capacitors in the op amp feedback to create active filters.


Try the below link for op amp info as applied to audio.
http://sound.westhost.com/

Mag748

C.W., G.Herzog,

Thank you for the resources. What I gather is that these capacitors are only for DC blocking, and low frequency attenuation. But with the capacitor values I am using, the frequency cutoff will be below what I would require, so as not to cause a problem.

Thanks,
Marcus

LoopyByteloose

Well, with the correct dual power supply to an op amp, you really don't need to block any DC. So you can direct couple without the capacitor. This is done all the time in audio preamps that might used about four op-amps in sequence to retain go linear gain.

Try that web side I mentioned. There is a lot to learn and a lot of helpful info there. Texas Instruments wrote a big book that is free on the internet on proper use of op amps.

BTW, It is NOT uncommon for the power feeds to have decoupling capacitors. And the amplifier stages can either have or omit depending on design goals.

Tracy Allen

The 150Ω resistor in series with the 22µF capacitor may be an important factor in the stability of that circuit. The resistor provides for phase lead to compensate for phase lag that is due to the output resistance of the amplifier driving the capacitor (an RC circuit) added to phase lags through the amplifier itself. The resistor provides a bit of feedback that is proportional to the current into the capacitor. Current leads voltage.

The problem is more acute with micro-power op-amps that have relatively high output impedance. It should be less of a problem with an op-amp like your NE5532, which has high drive capability.

A direct coupled high gain amplifier will usually need overall feedback at DC, to prevent DC offset errors from accumulating and saturating at the output.