Opamp in class A?

Ryssen · 29th February 2004, 05:00 PM

I have been reading a little about biasing an opamp in class A,and as I understand it you have to use a resistor according to the output load,but what if i use different loads on the output for listening test and other,will a CCS solv this problem?And how do I construct this ? "never done sommething like this before"

Nelson Pass · 29th February 2004, 05:50 PM

You can buy a monolithic current source, or you can make one
easily enough. Try a resistor first, just for kicks.

For Class A, usually you want the bias to be as high as the
current to the load. For a resistor, this is easy - just use a
value (usually to the negative rail) the same value as the load.

Usually something like 10 K will do the job fine.

stappvargen · 29th February 2004, 06:28 PM

Maybe this link will be of some help:

How to bias opamps into class a operation

/Niclas

Christer · 29th February 2004, 06:29 PM

Since the person asking the question was from Sweden, just one
word of caution. Those monolithic current sources (constant
current diodes) that are easily available in Sweden (ie. at Elfa)
are spec'd for a minimum voltage drop of 10V. Hence they may or
may not work depending on the op amp supply voltage and the
signal amplitude. Discrete CCSs can be built to work with smaller
voltage drops.

PMA · 29th February 2004, 06:40 PM

Although it depends on OpAmps circuitry, the current source (sink) from output to -Vs (negative supply rail) does usually the job.

29th February 2004, 05:00 PM	#1
Ryssen diyAudio Member Join Date: Dec 2003 Location: Sweden--> Here	Opamp in class A? I have been reading a little about biasing an opamp in class A,and as I understand it you have to use a resistor according to the output load,but what if i use different loads on the output for listening test and other,will a CCS solv this problem?And how do I construct this ? "never done sommething like this before"

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29th February 2004, 05:50 PM	#2
Nelson Pass The one and only Join Date: Mar 2001	You can buy a monolithic current source, or you can make one easily enough. Try a resistor first, just for kicks. For Class A, usually you want the bias to be as high as the current to the load. For a resistor, this is easy - just use a value (usually to the negative rail) the same value as the load. Usually something like 10 K will do the job fine.

29th February 2004, 06:28 PM	#3
stappvargen diyAudio Member Join Date: Sep 2002 Location: Göteborg	Maybe this link will be of some help: How to bias opamps into class a operation /Niclas

29th February 2004, 06:29 PM	#4
Christer diyAudio Member Join Date: Sep 2002 Location: Sweden	Since the person asking the question was from Sweden, just one word of caution. Those monolithic current sources (constant current diodes) that are easily available in Sweden (ie. at Elfa) are spec'd for a minimum voltage drop of 10V. Hence they may or may not work depending on the op amp supply voltage and the signal amplitude. Discrete CCSs can be built to work with smaller voltage drops.

29th February 2004, 06:40 PM	#5
PMA diyAudio Member Join Date: Apr 2002 Location: Prague, Kitakyushu, Fukuoka	Although it depends on OpAmps circuitry, the current source (sink) from output to -Vs (negative supply rail) does usually the job.

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