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RE: impedence of CCS mu output

Posted by Michael Koster (M) on April 7, 2010 at 22:56:52

In Reply to: impedence of CCS mu output posted by bobbyj on April 7, 2010 at 16:55:01:

This is a circuit I'm working on which I think is representative of your case.

The Zout of the mu-output is a function of the gfs of the MOSFET you are using, the value of the MOSFET source resistor (Rset), and the internal plate resistance of the tube. For a "large" value source resistor the value approaches that of a simple source follower of 1/gfs and is less dependent on the tube anode resistance. For finite small Rset values, the anode current is a fraction of output current defined by 1/(gfs * Rset). The Zout thus also includes Rp / (gfs * Rset) in series.

The gfs increases, therefore Zout decreases, with increasing current. The plate resistance also decreases with increasing current, resulting also in decreased Zout at higher current. Unfortunately, higher current means a smaller value Rset, which tends to increase Zout counteracting some of the improvement due to higher gfs and lower Rp.

Here is an example from the above circuit:

I'd like to run the 6C45pi at 5mA which means Rset of about 700 ohms with the CCS I'm using. This CCS has a gfs of about 20mA/V at 5mA, resulting in about 1/14 of the (dynamic) output current seen at the driver anode (thus adding about 1/14 the anode impedance to the CCS Zout of 50 ohms). The total Zout would thus be the 6C45pi Rp at 5mA (about 5K) divided by 14, plus 1/gfs of the MOSFET (50 ohms) so: (5000/14) + 50 or about 350 ohms + 50 ohms = about 400 ohms Zout as a very rough estimate.

I'm using a C1 of .21 uF so need 10,400 ohms R1. A 10K resistor plus the Zout of the CCS mu output will be close enough. Also 400 ohms driving a 10K load should be good. This also allows my D3A second stage to use a 500K grid resistor without loading the filter too much.

This is a work in progress so I don't have any measurements yet.

Michael

PS I haven't figured out whether to use bypassed resistors or diode bias yet. There will need to be something more than unbypassed resistors unfortunately (to keep the dynamic Rp low). Also "R2" in the EQ section should be 1K51 thats 1,510 ohms.

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Kimber Kable

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