Designing tuned amplfiers

In message , dated Wed, 9 Aug 2006, snipped-for-privacy@gmail.com writes

There is a book that goes into this in great detail:

Theory of transistor bandpass amplifiers, W. Th. H. Hetterscheidt, Philips Technical Library 1964. No ISBN at that time.

When it was written, no transistors were 'all that great', so it should be very helpful. These days, people use transistors that are 'great' enough not to worry about neutralization, still less unilateralization.

With regard to measuring transistor capacitances, there used to be a few pieces of test gear that would do this, a low-excitation voltage RF bridge with control over DC collector voltage and current, but I suspect there isn't much available now. You may have to build something.

You only need to neutralize to prevent oscillations. If you use adjustable inductors, you can align the oscillator by trimming.

The method is to trim the base side close to right, then do the collector and then go back to the base and get it exact. Trimming the collector will effect the base but trimming the base won't effect the collector (much).

With "not so great" transistors, low power gains may be all you can have.

Also remember that the impedances of a transistor change greatly if you run it class B or C.

So basically you recommend tuning out the parasitic capacitances with inductors? Another approach I considered was using transformers at the input and output to reduce the effective capacitance by n^2 and make then negligible. Does that sound feasible?

Another question I have is how can I accurately impedance match to the input of a common emitter stage? It seems easier for a common base stage since the input impedance is more predictable (1/gm vs B/gm for a CE stage).

Regards

Ken Smith wrote:

Yes, in the narrow band case, this works very well. You still have to make sure there isn't too much real component in the collector to base feedback. You don't want an oscillator.

Yes, but you give up a lot of power gain to do that. You will be driving the base with an impedance much lower than a matching impedance.

It can also make the problems on the previous stage worse. The loading of the collector of one stage bu the base of the next tends to reduce the chances of an oscillator.

At RF frequencies, the input impedance of a transistor is a complex matter. The "little Re" still has 25mV on it but you have lots of other stuff in the circuit.

For RF transistors, you can uses the maker's datasheet to find the input impedance in the intended operating mode. The value they give is just about the middle of the distribution.

If you want to find it out for yourself and are'nt working at too high of frequencies, you can make a circuit that allows you to adjust the impedance at the base. The simplest is a low valued POT from a signal generator.

In message , dated Fri, 11 Aug 2006, snipped-for-privacy@gmail.com writes

With 'not so great' transistors, you are liable to end up 'chasing your tail'; the thing will obstinately refuse to align and the bandwidth you get will be anyone's guess. You get 'throw-ups', too, at the pass-band edges, 10 dB or more, where the y12 feedback is nearly causing oscillation.

It's normal to use transformers, or tapped inductors or, less common but with notable advantages, capacitive tapping (you can vary the tapping by changing C values; you don't have to make new inductors). This is so that your tuned circuits have more practicable component values.

The input admittance of your CE stage is y11, and if you have values of it for your devices, you'll see that beta/gm is a very rough approximation. Because y11 is very much smaller than gm, you don't need such an extreme tap-down on your tuned circuit, so it's actually easier to match than a CB stage.

But the usual way to get stability with 'not so great' transistors is to deliberately mismatch at both input and output, so that the low source impedance and low load impedance keep the transistor from oscillating.

This is all explained in very great detail in the Hetterscheid book I pointed you at.