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Old 20th March 2013, 11:45 AM   #1
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Default CCS fixed bias

Hello,

Some time ago, Rod Coleman suggested using a CCS to obtain a clean fixed bias by generating a voltage drop on the grid resistor - the bias power supply and its capacitors are isolated by the constant current sink and are no longer in the signal path.

Great idea! I tried it in my R-C coupled GM70 SE amplifier. The schematic is below.

The bias PSU is well filtered by a MOSfet follower buffered voltage reference, and I tried two different constant current sinks: a cascoded, LED biased bipolar junction transistor CCS, and a 10M45S-DN2540 Mosfet cascode (which works down to 0.5mA current. A DN2540-DN2540 cascode couldn't go below 5.5mA in my tests).

The result - well, this CCS bias definitely sounds better to me than the classic fixed bias! I was very happy, until I checked the signal on the scope...

The negative going swing has its top soft-clipped!!!!! This only occurs after 18v peak swing with the BJT CCS and after 40v peak with the depletion mode MOSfet CCS. I hadn't heard the distortion because I only listened at a reasonable power level - I have sensitive speakers and neighbours...

Please see the oscillograms below. It's not a hard clipping, the waveform distortion appears at a certain level and remains quite the same up to the maximum swing (120v peak with D3a).

I can't figure this one out - what is causing this distortion and ruining a promising bias topology? (note: everything works perfectly with the classic fixed bias. The load is a 68K resistor as shown in the schematic.)
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File Type: jpg IMG_0289.JPG (93.7 KB, 559 views)
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Old 20th March 2013, 11:48 AM   #2
SY is offline SY  United States
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Are you running into compliance limits of the CCS?
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Old 20th March 2013, 12:22 PM   #3
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If the volt/div is 10V on the pic, that looks like the cause. Bias set at -110V, Vneg is -140V, giving less than 30V available.
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Old 20th March 2013, 12:23 PM   #4
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Hmm.. .that may explain it... the bias PSU is only 140v...

Pretty obvious, but I didn't see it... thanks guys.
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Old 20th March 2013, 12:28 PM   #5
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Old 20th March 2013, 02:08 PM   #6
DF96 is offline DF96  England
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In what sense is it an advantage to add a (necessarily imperfect) CCS in parallel with the 'signal path' instead of merely having a low impedance voltage supply at the other end of the grid resistor? As you have found, it makes things worse.
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Old 20th March 2013, 02:17 PM   #7
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I think the idea is that the high impedance CCS is not 'seen' by the signal compared to the relatively low ohmic resitor to gnd. This vs a fixed negative voltage that represents a stiff node and signal returning thru the bias network capacitors.
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Old 20th March 2013, 03:04 PM   #8
DF96 is offline DF96  England
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Not much signal gets to return through bias network caps, unless the bias resistor is unusually low in value. In any case, a typical cap is nearer to being an ideal capacitor than a typical CCS is to an ideal CCS. The other end of the CCS also needs a negative voltage supply of about twice the value needed for the normal voltage bias.

Most CCS have a limited voltage range, beyond which they can act as a clamp (i.e. voltage source). However, even given a perfect CCS I can't see any real advantage. Much better to have a voltage source at the other end of the grid resistor.
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Old 20th March 2013, 03:12 PM   #9
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Quote:
Originally Posted by DF96 View Post
In what sense is it an advantage to add a (necessarily imperfect) CCS in parallel with the 'signal path' instead of merely having a low impedance voltage supply at the other end of the grid resistor? As you have found, it makes things worse.
Actually it made things better... until it ran out of headroom, because of my own flawed design.

The advantage is, as SemperFi said, that the CCS has a very high impedance (tens of Mohm) at audio frequencies, and the signal no longer returns via the bias supply capacitors.


(another good fixed bias solution would be a shunt regulator like Salas' SSHV, I think... )
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Old 20th March 2013, 04:39 PM   #10
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I would worry a lot more about a CCS (right at the signal) than a bias supply cap (remote from the signal). Remember, passive components are a lot closer to ideal than any active component. Those who suffer from faradophobia have, like all phobics, to add unnecessary complications to their lives in order to avoid the feared object. Even a nasty bias cap will produce very little distortion at the valve grid because of the low output impedance of the previous stage.

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