I finally have much better results! The Q1 input transistor makes all the difference. I've been trying various Q1's and have found several that work very well. Low end input impedance is now under one ohm. The same 2N5109 Q2 is used; I haven't tried changing it (Central Semi, DC beta 110). I've been looking at the 1 MHz point for my Q1 comparisons. Best is 1.05 ohms with an obsolete Motorola 2N4401; a metal can BC108B gives 1.07 ohms; a metal can 2N3947 gives 1.10 ohms; an obsolete Motorola PN2222A gives 1.11 ohms.
50 kHz 0.86 ohms input impedance
1 MHz 1.05 ohms
10 MHz 2.93 ohms
20 MHz 5.33 ohms
30 MHz 7.79 ohms
Steve
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On 9/17/2022 2:58 AM, Tom Lee wrote:
If one performs a more detailed analysis, a good approximation for the output resistance should be:
(rb2/beta2) + re2 + (1/gm2) + ESR
I don't know the parasitic resistances of the transistor, unfortunately, so I'm forced to guess. But a good RF transistor should not have large base resistances. I'll make up a number, 25 ohms, which is higher than I think is reasonable, partly to compensate for neglecting Q1's contribution to the base resistance of Q2 (a few ohms). For beta2, I'll use the universal guess of 100, so the first term contributes 0.25 ohms. You can substitute your own values in the equation above.
The parasitic emitter resistance should similarly be small; I'll conservatively guess 0.25 ohms for the 2N5109. The actual value is likely half that.
At a current of 84mA, 1/gm2 is around 0.3 ohms.
You measured a capacitor ESR of 0.26 ohms, so adding up all the terms gives us a grand total of an ohm or so. Closer to your measured value, but...
--Tom
