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Old 10th February 2012, 05:49 AM   #21
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I agree, that's been some time since a simple linear amp has made me think more than 1 minute to figure out what's going on
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Old 10th February 2012, 08:34 AM   #22
Elvee is offline Elvee  Belgium
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Quote:
Originally Posted by PRR View Post
It will work exceptionally well for the number of parts.
NFB is small, especially through crossover....
The auto-bias scheme scheme can be applied to more "evolved" designs, like the one proposed by John.

In this minimalist forms, it suffers from some minor deficiencies: although it does not suffer conventional crossover distortion, there are some "crossing artifacts" caused by the time taken by Q3 to get out of deep saturation.
Adding an antisaturation diode improves matters.
Using one or two diodes to reduce the swing seen by Q3's collector helps iron out the last residues.
Adding the diodes somewhat reduces the bullet-proof thermal ruggedness of the circuit, but even with two diodes, the voltage across the bases is bound to VD2+VD3-VD1, which means there is still ~1/2Vbe safety margin, quite sufficient for a "normal" environment.

Further sophistication is possible, to improve some parameters, like adding a second transistor to make the circuit symetrical, add B-E resistors, a holding capacitor, etc, but the circuit then loses some of its appealing simplicity.
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Old 10th February 2012, 11:59 AM   #23
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One thing I notice is that one output transistor never turns fully off.
The other turns completely off. If you add one NPN transistor, base
to base with Q2 and collector to collector with Q3. You get the AND
logic function (in current) from those series collectors. Unfortunately
it switches with some hysteresis and/or has no fuzzy threshold, but
it does permit both outputs a non-zero minimal current at all times.

With base to emitter resistors, you can further up current minimum.
Wish I could fuzz out that logic for a little bit smoother crossing.

Last edited by kenpeter; 10th February 2012 at 12:11 PM.
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Old 10th February 2012, 02:15 PM   #24
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Even when ironing out the "wiggles" with all three diodes, HF distortion still isn't great. For a 28 mVp, 10 kHz input I'm getting like -28 dB 2nd. Not good "first (milli)watt" performance, I'd say.

A conventional 3-transistor, 2-diode circuit appears to be doing quite a bit better in this regard.
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Old 10th February 2012, 02:52 PM   #25
Elvee is offline Elvee  Belgium
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Quote:
Originally Posted by sgrossklass View Post
Even when ironing out the "wiggles" with all three diodes, HF distortion still isn't great. For a 28 mVp, 10 kHz input I'm getting like -28 dB 2nd. Not good "first (milli)watt" performance, I'd say.

A conventional 3-transistor, 2-diode circuit appears to be doing quite a bit better in this regard.
In fact, it has little to do with HF: the behavior is similar at low frequency.

As I said earlier, the audio quality is not very good, the H2 issue is caused by a gain change between the two situations: Q3 in linear mode/Q3 saturated.


This may look very surprising, but even used "backwards", Q3 provides some gain to the output.
Trying to follow the "gainy" signal path is an interesting intellectual exercise.

Anyway, there are possible workarounds.
The most obvious is to degenerate the negative half, see below.

B-E resistors can also be added to artificially raise the bias level, etc, etc
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Old 10th February 2012, 02:54 PM   #26
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Originally Posted by john_ellis View Post
Are you still there?
Here's a simple output stage I used for a tranny radio. Works well.
John
Hi John,

I particularly like your circuit for its simplicity and use of small transistors. Do you know the overall quiescent current drain please?

Should Q5 be mounted in contact with one of the output devices?

Regards,
currentflow
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Old 10th February 2012, 06:59 PM   #27
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Looking at the schematic, quiescent current as shown should be about 9 mA (mostly VAS Ic) plus whatever you choose for the output stage. This circuit is on the low complexity side of things, so for decent performance it can't have super low current (one of the typical "pick two" situations).

It certainly can't hurt to mount Q5 like that.

It should be possible to add more output device pairs, but eventually compensation will be required.
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Old 11th February 2012, 08:20 AM   #28
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Quote:
Originally Posted by sgrossklass View Post
Looking at the schematic, quiescent current as shown should be about 9 mA (mostly VAS Ic) plus whatever you choose for the output stage. This circuit is on the low complexity side of things, so for decent performance it can't have super low current (one of the typical "pick two" situations).

It certainly can't hurt to mount Q5 like that.

It should be possible to add more output device pairs, but eventually compensation will be required.
Hi sgrossklass,

Thanks for your reply. I came to a similar conclusion when calculating the VAS current, which seems relatively high considering the amplifier's application, particularly if battery-powered. As you say, a compromise has to be made between current drain and performance.

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currentflow
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Old 11th February 2012, 10:21 AM   #29
Wakh is offline Wakh  Russian Federation
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may be to try such a modification of scheme?

* - D1 is opened at negative half-wave output signal
and closes at the positive half-wave.
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Old 11th February 2012, 12:11 PM   #30
Wakh is offline Wakh  Russian Federation
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Sorry,
C3 = C4 = 1000 uF 16 V

Regards.
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