Class-A Opamps

[KSTR]

Originally posted by lineup
To single end bias the output stage of one Class AB op-amp
will not give you one Class A op-amp.
Because the op-amp was designed & calculated for a specific output stage operation
= Symmetrical Class B / Class AB, in 90% of all known cases.

This is completely unsubtantiated.

A class-B (typical) opamp that is designed for, say, +-30mA output current will happily provide +-10mA load swing either in class-B (-10mA...+10mA) or in class-A (+5mA...+25mA). Except for the output stage the rest of the internals are usually class-A in most IC opamps, only some micropower specialist types may have class-AB driver stages etc.

Some OpAmps even provide true "class-A" output pins (the comp. pin of the venerable 5534, ie).


@EWorkshop1708: The MC33079 is a "known good" quad audio opamp, and probably the best in price/performance ratio in the bipolar category. 33078's/33079's are used in many upper class hifi and studio gear.

- Klaus

[OPAQ]

I measured the bias current of the final stage of Burr-Brown's opamp OPA134. See next post.

[OPAQ]

I add a picture with this post, but I doubt whether it will be placed. What I did is measured the current in the positive and negative supply pins of the OPA134. The supply was +10.5V -10.5V. I used a harmonic signal of 200 Hz and 34.6mV effective voltage. The output was shorted with a resistance of 165Ohm.

I visualized the current through the supply pins with an oscilloscope. I increased the signal amplitude till the harmonic current signal in the supply pins begins to look distorted.

The maximal current (amplitude of harmonic current) turns out to be 0.3mA. So I conclude that the bias current in the final stage of the OPA134 is about 0.3 mA.

If you want to know more details, let me know.

[Workhorse]

Bump 1

[tiefbassuebertr]

Originally Posted by Workhorse

Hi everyone, Has anybody came across Class-A opamps?. Do they really exist.
Does there is a way in which a regular class-AB opamp could be converted to Class-A opamp. waiting for replies....... Regards, Kanwar

The actually question must be follow: How much is the DC current (bias) through each stages?
Most opamp datasheets offer only the whole DC current so as sometimes the spread aera. But most op-amps contain basically two voltage gain stages so as SE/PP-BUF (top quality opamps uses only one voltage gain stage - mostly as folded cascode like AD797 or OPA2604/2134).
I want to know the idle current of each stage.
BTW - each stage of all OP-Amps works always in pure class A (also the output stage), if the used AC/DC output current is below the quiescent current.
The idea from post #72 and #73 is very good (thank you therefore), but I would like it, when the manufacturer-datasheet would be offer accurate data in this matter.

[AndrewT]

I don't think so.

let's use a simplified example of an opamp that has the 3 stages of the Lin topology and the last stage is push pull.

The first stage draws Quiescent current Iq1
The second stage draws Quiescent current Iq2
The third push pull stage draws Quiescent current Iq3 = Ibias

Let's further assume that no other quiescent current is drawn by auxiliary circuits.
Iq1 & Iq2 could be feeding single ended ClassA stages.
The total quiescent current (Iq) is Iq1+Iq2+Iq3
The maximum ClassA output of the Lin amp is 2*Iq3
We have many combinations of varying current in the 3stages as the output current is increased towards IclassA max.

One must account for Iq1 & Iq2 before one can draw conclusions about IclassA max.

Then the opamp will have a non zero Iauxiliary. More complication

Opaq's method relies on the not unreasonable assumption that the rail currents of a ClassA amplifier follow the ClassA output current.
It appears he has detected the change in rail currents to include the half wave rectified versions of the ClassB output current.

[Workhorse]

Better to make a diy Class-A opamp with SMD parts which can fit on a custom built DIP 8 or 16 adapter

[tiefbassuebertr]

Originally Posted by AndrewT

I don't think so.

let's use a simplified example of an opamp that has the 3 stages of the Lin topology and the last stage is push pull.

The first stage draws Quiescent current Iq1
The second stage draws Quiescent current Iq2
The third push pull stage draws Quiescent current Iq3 = Ibias

Let's further assume that no other quiescent current is drawn by auxiliary circuits.
Iq1 & Iq2 could be feeding single ended ClassA stages.
The total quiescent current (Iq) is Iq1+Iq2+Iq3
The maximum ClassA output of the Lin amp is 2*Iq3
We have many combinations of varying current in the 3stages as the output current is increased towards IclassA max.

One must account for Iq1 & Iq2 before one can draw conclusions about IclassA max.

Then the opamp will have a non zero Iauxiliary. More complication

Opaq's method relies on the not unreasonable assumption that the rail currents of a ClassA amplifier follow the ClassA output current.
It appears he has detected the change in rail currents to include the half wave rectified versions of the ClassB output current.

what this refers ?? (post #??)
The term "non zero Iauxiliary" I haven't heard until this day. Sorry for my bad english.
Mean you the necessary base input current value of the follow stage?
This must be much more lower than the quiescent current of the previous stage.

[dimitrij]

class A biasing for opamps seems a nice idea. I will give it a try
for my new preamp that i'm gonna build soon.

A (maybe) stupid question: will it work also for power opamps?
I'm talking about LM1875 and similar devices.
Has anyone tried this yet?

[mt490]

The LM1875 already boils so I'm not sure that's really a good idea with its TO-220 package.

Remember the dissipation in meaningful Class A bias goes up quite a bit due to the squared relationship.

Plus with the LM1875 in particular the protection circuit might not be very happy with what it senses.