Re: Speech processors...

On Wed, 19 Sep 2007 12:56:19 -0700, Telstar Electronics
<briangriffey@xxxxxxxxxxxxx> wrote:

|On Sep 19, 2:32 pm, james <geo...@xxxxxxxxxxxxxx> wrote:
|> Read the text that describes the circuit.
|
|Nowhere in the text does it talk about the function of D1 & D2...
|www.telstar-electronics.com
|
|
Brian

Correct. The diode function is intuitive. One should very easily
recognize the voltage doubler that D1 and D2 perform. One should also
understand from his minimal text that any diodes used in a feedback
loop are not for "clipping" but to rectify an AC voltage to produce a
DC voltage. From reading the text it should be quite obvious that the
circuit is a compressor. You do not need feedback loops in a clipper
stage. You do need feedback to control gain as in a AGC circuit or a
compressor.

Voltage doubling techniques using two diodes if very well covered in
technical training. But I will tell you this that D1 and D2 form a
half wave voltage doubler.

here is a link that may help you and your good buddy audio to
understand what a half wave voltage doubler is.

http://www.tpub.com/neets/book7/27m.htm


Furthermore this doubler feeds tothe base of Q2 which acts as an
inverter to feed the inverted rectified audio voltage to the gate of
the FET Q2. Q1, the FET, acts as a variable resistor. Q1, MPF102, is
a N channel depletion mode Junction Field Effect transistor (FET). N
mode depletion FETS require the gate voltage to go positive with
respect to the source voltage. Once the gate voltage goes below the
source potential, the device turns off. The the path from drian to
source approaches a very high resistance, approxiamately 1 megaOHm.
When the gate voltage is higher than the source voltage then the drian
source path become a very low resistance. The actual resistance
depends more on the channel widths of the device but usually in the 1
to 10 Ohms range.

So D1 and D2 form a voltage doubler that takes the output from the op
amp thro ugh C6. This voltage is 100 times what is fed to the radio.
Rectifies it so that it is now a DC voltage at ~200 times the radio
voltage. That DC voltage is fed to Q2, and common bipolar transistor,
which is in a common emmitter configuration. That inverts the DC
voltage fed to the MPF102, FET.

Brian, you profess to be very knowledgable. Even a average tech from a
reputable tech school or even with military training should be able to
recognize how this circuit works. It is rather simple and straight
forward. The more you post just further convinces me that you have no
more skill than the average screwdriver techs that proliferate the CB
band.

james
.

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