VAS with CD-CB stage

[x-pro]

I've used this VAS in several production amplifiers around 1995-1997 and thought it would be useful to share this idea and to see if that arrangement have been used before - perhaps somebody would remember. The idea is to connect a common drain into a common-base stage, effectively creating a voltage to current convertor. Advantages are - good speed and linearity, completely symmetrical voltage and current limiting on the output, high OL gain in a 2 stage amplifier. On the first diagram here is a simplified configuration, on the second - a complete and known to work circuit with BJT output. However I've used this VAS with MOSFET output stage as well.

x-pro

P.S. - transistor types on the second diagram I had to choose from available models to get the simulation to work. However BC550/BC556 for small signal, BD139/140 (Philips) in place of MJE15030/31 and 2SC2922/2SA1216 in the output were originally used. P-channel MOSFET could be ZVP3306, ZVP3310 or similar.

[AKSA]

Looks good, neat constant phase circuit, very high bandwidth which would have to be tamed to meet Bode/Nyquist criteria, but is there really an advantage in using a Pmos, why not a bipolar?

How does it sound? Soundstage? Fine detail? Low distortion?

Cheers,

Hugh

[darkfenriz]

Originally posted by x-pro
... The idea is to connect a common drain into a common-base stage, effectively creating a voltage to current convertor. ...

aka. long tail pair ??

[AKSA]

Adam is right; it mimics the LTP input stage, which is an emitter follower feeding a common base connection..... (though the second base 'moves' as its the inverting input)

Cheers,

Hugh

[CBS240]

Originally posted by darkfenriz



aka. long tail pair ??

with the -input as V(D4)...good one

[x-pro]

Originally posted by AKSA
Looks good, neat constant phase circuit, very high bandwidth which would have to be tamed to meet Bode/Nyquist criteria, but is there really an advantage in using a Pmos, why not a bipolar?

Pmos gives the VAS an extremely high OL gain - over 120 dB into 10K load without much complications, and very good linearity, as it is essentially a voltage to current convertor working on a small part of the MOSFET transfer curve. As the gain in the first stage is high, the distortion are mainly 2-nd and lower 3-rd harmonics from the MOSFET, and even these are at a very low level.

Originally posted by AKSA
How does it sound? Soundstage? Fine detail? Low distortion?

It sounds good . I've used this VAS configuration in 5250 integrated amp and A52/A52SE power amplifier from Creek Audio (thought in A52 I've made the input diff. stage from a matched and cascoded JFET pair and the output was N-ch MOSFETs on both) and another good example of this VAS was A3i amplifier from Cambridge Audio which I've designed about the same time, using BJT output transistors (from Sanken). All these amps had a very good sound (IMHO).

Cheers

Alex

[x-pro]

Originally posted by AKSA
Adam is right; it mimics the LTP input stage, which is an emitter follower feeding a common base connection.....

Yes and no . It looks like an LTP, however the input impedance of the BJT in CB configuration is much lower that the output impedance of MOSFET in CD at about the same current. Because of this imbalance the output impedance of the BJT goes higher, the transfer curve looks like a single-ended FET stage and as the voltages on the FET are not changing much - quite a linear stage too.

Cheers

Alex

[AKSA]

Greetings Alex,

You are a sleeper - an ex-Creek designer, I had not realised.

No bloody wonder.....

I always try to design the VAS with a naturally falling FR to help with compensation. To this end bootstraps are useful, so is the common emitter configuration. Your use of common base, driven in common source at a very high impedance gate (yielding very high gain for the input stage), would appear to give stratospheric top end, very little phase shift (good), and 120dB OLG into 10K (current dependent of course) is also very good.

What distortion is introduced would be presumably low order, particularly H2/H3 from the mosfet as its current would vary very slightly.

What more can be said? It's very good..... congratulations!

Cheers,

Hugh

[x-pro]

Originally posted by AKSA
Your use of common base, driven in common source at a very high impedance gate (yielding very high gain for the input stage), would appear to give stratospheric top end, very little phase shift (good), and 120dB OLG into 10K (current dependent of course) is also very good.

What distortion is introduced would be presumably low order, particularly H2/H3 from the mosfet as its current would vary very slightly.

Your analysis is quite correct (thought it is common drain for the MOSFET).

Thank you!

Alex

[Bonsai]

Alex, in your circuit, I see C4 and C7 which are effectively capacitice shunts to ground, and then the same again with the C3+R13 network. What effect do these have on the OL gain response? Are these not heavy loads to tame the loop response?

Very interesting VAS topology BTW.