[Boatanchors] Programmable Osc Setback

[KA4INM radio]

   On 11/15/14 10:41, David Stinson wrote:

> OK, you fellas can "I told ya so."   I won't complain.
> On the receiver side, the tiny output of the programmable
> oscillators work fine and I hear no problem with increased noise floor.
>
> Not so on the transmit side, were the osc needs
> considerable "boost" to drive the 6L6 grid.
> My attempt to keep the TX Xtal-replacement circuit ultra-simple just
> isn't up to the job. The output is noisy and has unacceptable spurs.
> Not about to give up on it- I have high hopes it
> can be made to work in the BC-375.

> There's a couple of ways we might go from here.
> The input impedance of the MOSFET gate is quite high,
> so a Hi-Q tuned circuit between the OSC and Gate
> might bring the nastys down to something workable.
> Might also turn the FET into a oscillator itself, so a tuned buffer as
> has been suggested might be in order.
> Can you neutralize a MOSFET? *
> Operating the MOSFET linear might also help. *

> Ideas?


*  The most common kind of MOSFET transistors are "depletion mode" that 
require being biased 1/2 off (UN-LIKE bipolar transistors) for linear 
operation, the other kind is the enhancement mode MOSFET transistors 
that starts out just like vacuum tubes, cut off or with a minuet amount 
of channel current and enhancing the gate Voltage (toward the drain 
Voltage) increases the channel current to the point where the current 
can go up and down in the linear portion of the device's "transfer 
characteristics". (just like in most vacuum tubes)
   There are N & P channel MOSFET transistors, much like a bipolar 
transistors (NPN & PNP) that act the same way. (supply is + or -)

  The link ( home.netcom.com/~arc5/oscbrd.jpg ) properly shows an 
enhancement mode MOSFET transistor, but using a power size, a 2N7000 (in 
a TO-92 case) would have been a better choice in my opinion.  There is 
no forward bias in the link diagram bringing the MOSFET into the linear 
region of operation.  This should help.

   (Handling the 2N7000 can damage it, puncturing the gate insulation, 
wind a very fine (wire wrap wire?) between the three leads {at the body} 
while the lead tips are held together then remove it after soldering the 
transistor in place.)

   An enhancement mode MOSFET transistor is like a triode and a dual 
gate MOSFET is like a tetrode, they are linear amplifiers unless driven 
into non-linear reasons of operation.  If you stack two enhancement mode 
MOSFET transistors (drain to source) you create a tetrode type 
transistor.  This reduces the miller feedback and usually makes 
neutralization less necessary.  The upper gate must be held at RF 
ground, but biased up into it's linear region.

   If necessary MOSFET transistors can be neutralized. It might be 
better in this case to swamp the input of the enhancement mode MOSFET 
transistor with a "swamping load" (without altering the bias on the FET) 
with perhaps 1,000. Ohms, this generally renders the miller feedback 
ineffective.  (preventing oscillation)  Stability can be tested with the 
oscillator disconnected.  (with the swamping load connected)
-- 
   Ron  KA4INM - Youvan's corollary:
                 Every action results in unwanted side effects.