# FET Mixer

Discussion in 'Electronic Design' started by redhat, Jun 21, 2005.

2. ### Jim ThompsonGuest

Calculate the gate voltage resulting from the voltages at IP1 and IP2
and then you'll understand. (There is no current into the gate.)

...Jim Thompson

....and then pickup a copy of the Art of Electronics if you want to calculate
what the gain is going to be. (Hint: It's approximately R4/R3)

4. ### JoergGuest

Hello Joel,
....and don't forget that the FET needs a DC bias. Capacitively coupling
both audio sources into the resistors would not be a good thing here...

Regards, Joerg

5. ### Helmut SennewaldGuest

Hello "redhat",

There is a big misunderstanding about the word "mixer".
When people coming from audio talk about a mixer, then
This means the circuit adds the signals of two sources.
It can be done in the most simple way with just two
equal resistors as it's done in this circuit.

When we can neglect the source resistance of the audio
sources then the result is:
Vout=(V1+V2)/2

The FET in this schematics just acts as a buffer to get a low
output resistance. It could be replaced by any kind
of transistor or an opamp if some small circuit modifications
will be done.

Best regards,
Helmut

6. ### Tim WescottGuest

I'd be quite inclined to capacitively couple into those resistors, then
add a third one to a bias source of _my_ choosing, rather than depending
on some unknown audio input to have something suitable.

7. ### JoergGuest

Hello Tim,
That is how I would do it as well. Unless the inputs are XLR transformer
coupled and the xfmrs are on the circuit.

Regards, Joerg

8. ### Pooh BearGuest

Quite badly I suspect ! Ppl who are clueless realy shouldn't post their
ignorance on the net.

Graham

9. ### Pooh BearGuest

Please *do* explain how R4 and R3 affect the gain !

Note it's an n-channel fet. R4 is simply a waste of space.

It's basically a source follower Gain =~ 1 with an input attenuation of 6dB (
presume R1 = R2 ) caused by the passive summing.

The output will need to be ac coupled too. I won't even start on the
indeterminate input biasing.

Graham

10. ### Jim ThompsonGuest

Graham, That's rather rude. Didn't get any last night ?

...Jim Thompson

11. ### martin griffithGuest

doesnt stop me......

martin

12. ### Tam/WB2TTGuest

He is right, though. Bad mixer, but maybe OK combiner if he puts some bias
on the FET. I am somewhat concerned about one source back driving the other
one. Op amp would be better, unless he is going to build 8 million of these.

Tam

13. ### Jim ThompsonGuest

I wasn't commenting on the mixer, just the "Ppl who are clueless realy
(sic) shouldn't post their ignorance on the net".

I have no problem with students who are asking a non-homework
question. There are no stupid questions, just stupid answers.

I DO have a lot of problems with rude, pushy, IGNORANT punks, or is it
pukes, like that TokaMundo.

...Jim Thompson

14. ### Pooh BearGuest

Pfftttt... ! Silly boy - lol . ;-)

Keep them coming.

Graham

15. ### Pooh BearGuest

It was mean to sound humourous actually, on account of how the question was
phrased. Sorry if it missed the spot. We have this rather 'dry' sense of
humour in the UK that you guys sometimes find trouble tuning in to it seems.
Different culture I suppose.

It *is* a truly awful example of an incomplete and bad circuit though !

Graham

p.s. and no I didn't. :-( Peggy was meant to ring back but got 'diverted'
it seems.

16. ### Tam/WB2TTGuest

You haven't run across Doctor Slick, have you ! .

Tam

17. ### Tam/WB2TTGuest

I gather you are trying to add/combine the two signals, not mix the
frequencies. Your schematic implies the inputs come from current sources; in
that case you want to have a buffer with a very low input impedance, like
the (-) input of an opamp, not a high input FET.

Tam

18. ### Jim ThompsonGuest

I probably already have him permanently kill-filed, just like
TokaMundo.

...Jim Thompson

19. ### martin griffithGuest

hehe, just having a bad day. Been trying to understand Avid Xpress
colour correction. I hadnt realised how soft some of my shots are, but
what a klunky counterintuitive program. And I thought some PCB design

At least "her next door" cooked me a nice curry

martin

20. ### Pooh BearGuest

Mea culpa. ( re: the sic )

I've always preferred the idea that the best spell checker is yourself. You may
understand this better if you've seen what Microsoft Word ( hissss - spit - spit
our the damn talking paperclip ) is capable of. I recall one instance where Word
had a hissy fit over something one of my colleagues had typed and 3 of us looked
at it for around 5 mins and couldn't for the life of us work out why it had
highlighted it - even including considering context !

I do however type ( should the be typo ? ) quite fast and inaccurately sometimes
and simply review the result and correct. I find this to be an effective way of
generating text over the one-fingered approach.

I regret that one slipped past the sub-ed's eye. ;-)

As for ppl posting shit on the net. I stand by my comment. Some ppl may read the
shit and actually believe it !

Graham