How To Make This Gain Stage Clip Nicely

[Samuel Groner]

Hi

Working on this one: [removed]

Everythings seems to be fine, except that clipping results in most ugly polarity reversion and a tendency to latch up; any simple fix for this?

Samuel

 

[cuelist]

Samuel,

I'm trying to understand this circuit;

- Why Q2/R2 instead of a plain 'ol resistor?

- What's the purpose of D6?

 

[Samuel Groner]

I think R1/Q3, not R2/Q2 could be replace with a resistor, but the value for the resistor seems to be rather critical for correct Q1 collector current and "low" offset, so I thought why not fix this current? Couldn't that make better PSRR as well?

D6 does usually improve recovery time from clipping, but here it has little influence. If there's no simple fix for the clipping issue, I'll skip it.

Samuel

 

[cuelist]

[quote author="Samuel Groner"]I think R1/Q3, not R2/Q2 could be replace with a resistor...Samuel[/quote]

Ooops! you're right of course, I didn't look carefully,

One change to consider would be to go for a 2-transistor current source. That gives you something like 10x higher impedance and if you split the biasing resistor and add a capactor you'll get a significant increase in PSRR.

Like this (disregard values, this is from another schematic);

http://tinypic.com/bgpbia.jpg

 

[tk@halmi]

Nothing clips nicer than a resistor current source. JFET current sources are pretty graceful also. Check it out on a scope. I used to play with that stuff for days. Putting tight current sources on everything will aid complex clipping characteristics. Believe it or not, even the JH990 exibits phase reversal when pushed hard. Why not go for the symmetrical input stage and reduce offset that way? Just add a few more transistors.

 

[PRR]

No load, no voltage gain.... why have an amp at all?

Unity gain is often worst-case for input overloads. The input has many other chores to do; don't expect it to swing rail-to-rail also.

Is your typical source really likely to swing that far? If so, put a 2:1 pad on the front, a gain of 2 in the amp, so the input stage does not have to work so hard.

 

[rickc]

BiFet opamps (TL07x) do this when you run them as voltage followers. The cause is exceeding the CM input voltage limit for the opamp. The cure is a clamp diode at the inputs.

Could you post your netlist for this (it looks like this was entered into a spice program); save me time entering the circuit so I can try some things.

 

[Samuel Groner]

Thanks for the answers.

cuelist, I don't think we get much by using more advanced current sources; PSRR seems to be dominated by other effects at mid and high frequencies, so no way around some RC filtering. But the additional CCS (R1/Q3) makes a vigorous 20 dB difference, which is nice to have.

Interesting to know that the CCS type has a major influence on clipping, Tamas. How does the GainBloake (is this the official spelling, BTW?) clip?

In the meantime I settled on just leaving as it is - I don't think there's a simple solution (like a clamping diode). Clipping is ugly at high gain as well, which indicates that it is not a simple exceeding of CM input range. So thanks for your offer rickc to do some work on this, but not necessary any more!

The amp is taken out of context here, four of them should once make my personal spin-off from Brad's transformerless mic pre wish list.

Samuel

 

[cuelist]

[quote author="Samuel Groner"]I don't think we get much by using more advanced current sources; PSRR seems to be dominated by other effects at mid and high frequencies, so no way around some RC filtering.[/quote]

Well it's up to you to try or not of course :wink:

I was only suggesting a solution. This alternative current source and the filtering has been mentioned in several sources including Doug Self's book on power amplifiers.

Incidentally, a lot of what's valid for power amps is also valid for signal amps like you are working on. I have found such books as Self's to be very good at explaining the finer points of discrete amplifier design.

 

[Samuel Groner]

Sure, I read Self now about three times and it is truly a great book.

In my design, the simulated negative PSRR improves about 40 dB at low frequencies with ideal current sources; above 1 kHz and for the positive rail there is essentially no change.

As I'll use a regulated PSU anyway (and thus low hum) I'm more concerned about high-frequency PSRR (for best stability and RFI-immunity) which we can only improve with RC filtering.

I basically agree with and appreciate your opinion - just want to explain my reasoning!

Another Q which came to my mind: Does the R2/Q2 CCS contribute significant noise in this design? I can't make my mind up - at the moment I'd vote for "no", but what do I know...

Samuel

 

[tk@halmi]

How does the GainBloake (is this the official spelling, BTW?) clip?

On Monday I will have time for some scope shots (pictures). The spelling is GainBloak (C) 2005.

 

[bcarso]

The "phase reversal" at clipping and other abberant clipping behavior comes from two places.

The "devil's horns" in positive overload is due to the Q4-Q5 stage and D6 causing the drop across the emitter resistor of Q5 to get much larger than when the loop is closed. This is because now both the normal ~9mA and additional current from Q1 are flowing. The output emitter followers faithfully reproduce the collector voltage of Q5, which more or less follows the voltage at the R5-Q5 junction.

On the other overload swing the thing clips not too badly, but notice the transient instability coming out of clipping. I think this is due to the overload state saturating Q6, which then pulls down on the D4-D5 bias and this nearly shuts down Q2.

Note that, if the output stage could swing a bit higher, on the positive swing the reverse protection diode D1 could get forward-biased and for a highish source Z induce latchup. But this can't happen unless Q5's emitter were sitting a lot lower, as it requires both D1 and the collector-base diode of Q1 to be forward-biased.

What are the fixes? You could contrive a different clamp mechanism for the Q4-Q5 stage (for example, a third transistor' emitter to +18V, base tapped down a split R5---but then you would have this actuate only after a delay with C2 there). You could make R5 something else, that biases the emitter as it is now but is stiffer, for example an LED or a couple of diodes, or a Q Vbe multiplier. For the negative swing problem I suspect it would help to separate the I source bias sources, or at least put an R in series with each base and run a bit more current through the diodes. Or, as suggested, use the two-Q I source for each.

You could hard-clamp the bases of Q8 amd Q9, but this will still lead to slow recovery due to C2-R5.

Or, you could just avoid clipping :green:

EDIT: Actually I don't think you need as much as 1.7 volts drop to the emitter of Q5 anyway. Q3's collector will pull to within about 0.8V of the rail, which should be plenty enough to operate the Q4-Q5 stage.

EDIT 2: Yes---tie Q5 right to the rail, put 1k or so in series with the base of Q6. Things clean up nicely and the clipping is almost symmetrical.

 

[Samuel Groner]

Thanks Brad for the explanation and solution.

Anyone on the noise contribution of the current source?

Samuel

 

[PRR]

No.

 

[Samuel Groner]

No noise contribution or no comment? :roll:

Samuel