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previous: Doug Gotcha... I didn't look closely eno... -- 6/15/2000 3:53 PM

Re: Next Puzzler

6/15/2000 7:53 PM
Jeff	Re: Next Puzzler I think I have the answer; I will explain it in text, and hopefully get a schematic up later to clarify. FIRST -- disconnect the 5k pot and 1k resistor from the circuit, and place these going into a current mirror. The current mirror uses 2 NPN transistors with their bases tied together, and their emitters tied to ground. The bias network goes to the collector AND base of the first transistor. The second transistor will mirror this current through its collector. SECOND -- tie the two 100ohm resistors of the diode circuit to the collector of the 2nd transistor in the current mirror. THIRD -- Place an 800ohm resistor to VCC where the bias network was at. I chose 800ohms based on a 9V supply -- the bias current varies from 1.5mA to 9mA; this would leave the 800ohm resistor dropping 1.6V to 7.2V, leaving the current mirror well within its linear range. (I haven't done power calculations -- this transistor might need a heat sink or we might need a high-side current mirror as well). FOURTH -- place the network in the feedback loop of an opamp using capacitors. One side hooks up to the two 100ohm resistors (and the current mirror); the other side hooks up to the diodes. This places them in parallel. FIFTH -- standard opamp distortion circuit, with drive pot DC connected in feedback path of opamp. I believe that this will bias the diodes, and provide a high-impedance for the opamp to drive. As I said -- I will try a schematic later. Comments????? -Jeff

6/15/2000 7:53 PM

Jeff

Re: Next Puzzler

I think I have the answer; I will explain it in text, and hopefully get a schematic up later to clarify.

FIRST -- disconnect the 5k pot and 1k resistor from the circuit, and place these going into a current mirror. The current mirror uses 2 NPN transistors with their bases tied together, and their emitters tied to ground. The bias network goes to the collector AND base of the first transistor. The second transistor will mirror this current through its collector.

SECOND -- tie the two 100ohm resistors of the diode circuit to the collector of the 2nd transistor in the current mirror.

THIRD -- Place an 800ohm resistor to VCC where the bias network was at. I chose 800ohms based on a 9V supply -- the bias current varies from 1.5mA to 9mA; this would leave the 800ohm resistor dropping 1.6V to 7.2V, leaving the current mirror well within its linear range. (I haven't done power calculations -- this transistor might need a heat sink or we might need a high-side current mirror as well).

FOURTH -- place the network in the feedback loop of an opamp using capacitors. One side hooks up to the two 100ohm resistors (and the current mirror); the other side hooks up to the diodes. This places them in parallel.

FIFTH -- standard opamp distortion circuit, with drive pot DC connected in feedback path of opamp.

I believe that this will bias the diodes, and provide a high-impedance for the opamp to drive.

As I said -- I will try a schematic later.

Comments?????

-Jeff

Replies:

Jeff Quicky Schematic at: -- 6/15/2000 9:10 PM