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zxcvb

- 10

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I'm building a MOSFET driver like this. The signal source is 7V peak-to-peak. Vcc is 12V. The push-pull stage is 2n3904/2n3906.(datasheet http://www.kynix.com/uploadfiles/pdf8798/2N3904.pdf)

I've tested the push-pull output before adding the transformer, and using a 50 ohm load, the output voltage is reduced by half, so I concluded that the output resistance of the push-pull stage is 50 ohm.

Then according to the MOSFET spec (IRF740), C_iss = 1400pF, at 13.5MHz I calculated the impedance to be 8 ohm. So I used a 2:1 transformer (therefore 4:1 impedance transformation) to match the load at the gate.

But now I get a flat signal at the Q3 gate, AC is almost non-existent. If I disconnect the MOSFET Q3, I get a good sine-wave. Can anybody help me find out what's the problem with this circuit?

C2 is a 104 capacitor which is practically a short at RF. The 2n3904 has a spec of peak 200mA current capacity, which I translate to be 2.5V for 1/4 period, which should get doubled by the transformer and is supposed to be enough to charge the gate to a observable voltage.