strange problem of my oscillator buffer

[cutedrummerboy]

hi all, i have a strange problem... i have successfully completed a colpitts oscillator. it is the vfo from a ham band transceiver called bitx20. this oscillator is up and running and tunable also at 4Mhz but the problem is the buffer portion. i used 2n2222 for the oscillator as well as the buffer. i checked the frequency after buffer and watched it is decently stable. but also watched the transistor of buffer is getting too much hot while i switch on the oscillator. i dont know why this happen. can anyone tell me, is it normal??



i attached the oscillator circuit. the red box is buffer portion.

 

[FvM]

The buffer is a class A amplifier, biased near to maximum transistor power dissipation. So we can surely say that the disspiation is by design.

 

[cutedrummerboy]

thank you FvM. but it is getting too much hot, i cannot touch its metal case more than 10 seconds. do you think it really normal??

 

[FvM]

I wonder if your implemnted circuit is different from the schematic. In the latter, the output transistor has 100 ohms emitter and supply series resistor each. So maximum power dissipation of the transistor ever achieveable is 180 mW with 12 V supply (6²/200). TO18 case to ambient thermal resistance is about 300K/W, so you can expect maximum 54 K overtemperature. This is hot but not excessive.

 

[chuckey]

Are you sure that the .01 MF decoupling cap with the 10 ohms in series is not short circuit?
Frank

 

[FvM]

Are you sure that the .01 MF decoupling cap with the 10 ohms in series 10 ohms in series is not short circuit

The output stage may be possibly overdriven, causing unwanted harmonics. But this doesn't change the maximum available DC power calculation.

 

[rohitkhanna]

I presume you are talking about Q7 in your schematic ? This is NOT a buffer stage... it is in fact a classic gain stage, though your transformer is connected differently from most.

Having said that, and to answer your question - you are correct, it is not normal. You should (1) double-check your circuit and components for correctness, and (2) put some load on that transformer - start with 1K and see what happens.