[sdiy] Some help with a current mirror, please

[Simon Brouwer]

Hi Jeff,

At 20:31 28-10-2005, Jeff Farr wrote:
>I'm working on troubleshooting my x0xb0x and I can't for the life of me 
>understand how a certian current mirror is operating.
>Here is a pic of the schematic: 
><http://www.ladyada.net/make/x0xb0x/files/mainboard_beta.png>http://www.ladyada.net/make/x0xb0x/files/mainboard_beta.png
>I'm looking at Q1-Q4 in the VCA section and while taking measurements I've 
>found that turning the gate on puts .5v at the collector of Q3 and 12v at 
>the collector of Q1.  However, reguardless of the 'gate state' the 
>collector of Q4 is always at 12v.  It's my understanding that the 
>collector of Q4 should be ~ .6v below the rail while current is flowing 
>and 0v while current is not flowing.  Perhaps I just don't understand the 
>mirror?

Q2 is connected at a diode, so the collector of Q4 would not go more than 
one diode drop (ca. 0.7 V) below +12V.

I wonder why there are two OTAs in parallel?


Vriendelijke groet,
Simon Brouwer.

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