# Issues designing a RC Mosfet osciallator for 8 Mhz sine wave

Joined Dec 29, 2020
2
I want to design a RC Mosfet for 8 Mhz sine wave so I started by designing a Mosfet amplifier with gain 29 and to check it I provided a sine signal with 8 khz frequency and it worked fine but when I increased it to 8 Mhz it was no longer inverted.( For amplifier asc and calculations file see first two attachments)
The amplifier circuit is below

So I proceeded to Design the circuit for 8 Khz frequency but there were no oscillations even though gain was 29 and Mosfet was inverting.
The compete phase shift circuit is below
If anyone could help me figure out
First how to get oscillations started for 8 Khz ( or for any frequency)
Second how to achieve oscillations for 8 Mhz and maybe explain why amplifier is not giving an inverted signal for 8 Mhz , your help will be greatly appreciated.

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#### DickCappels

Joined Aug 21, 2008
7,426
The fact that the 8 MHz signal is not inverted indicates that the gate to drain impedance (about 4k) is too low compared to the impedance of the drain circuit.

#### GeBJT

Joined Nov 3, 2020
19
I can see a couple of things wrt the 8kHz version without having to fire up the simulation:

1] in the amplifier (only) cct the value of the load R4 is 1k42; in the phase shift cct R4 is 10x lower at 142. Intuition tells me that a load resistor much smaller than the drain resistor will significantly squash the gain!

2] back to the amplifier (only) cct: there is no load representing the RC network. R4 cannot do this loading function because it is resistive only. There will be only a small difference in gain when including the 3x RC "ladder", but that can be the difference between oscillations, or not.