EMRFD Message Archive 3653

Message Date From Subject
3653 2009-10-24 15:31:13 chrisethompson Trouble with EMRFD amplifier
Many thanks to the advice I got a while back with the "First Transmitter", which I eventually got working. I'm now struggling with another problem now.

I have built the Class AB amplifier Figure 2.93 in chapter 2 of EMRFD. I'm driving it from a T2 circuit with a VFO that I constructed on copper clad (VFO in a separate box). The T2 seems to be working well, with 0dBm out. However, I can't get the small amplifier to work.

The driver stage seems to produce abut 15mW when I disconnect it and measure the RF across a 50ohm resistor. The 2N3866 produces no RF. I've removed the filter, so the load is a 50ohm resistor.

To troubleshoot this I separated the base bias by lifting the 3k resistor (I used 3.3k in fact with a 330ohm to ground) and then measured the collector current at 74mA and the base bias current at 3.24mA. Supplying more current to the base using a 2.2k resistor instead, drove the DC collector current up to 157mA. So the transistor seems to be switching on. With no base bias, there is no collector current. Why does it not amplify RF? Any thoughts?

I've already changed the device once btw.

3693 2009-10-30 11:48:11 chrisethompson Re: Trouble with EMRFD amplifier
I solved this problem. The resistor I was using for the emitter was 1W and must have been wire wound. Hence the bias conditions seemed fine yet the signal was not amplified because there was a very high impedance path to ground through the emitter.