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Old 9th Mar 2018, 12:44 am   #9
G0HZU_JMR
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Join Date: Sep 2010
Location: Cheltenham, Gloucestershire, UK.
Posts: 3,077
Default Re: Testing unknown transistors for max frequency response

Yes, having a 3900 ohm resistor in the collector will limit the bandwidth once you try and probe the circuit with anything.

Ideally you would also want to measure the Ft of the transistor. I tried to think of a quick and dirty way to do this and the circuit for this is below. It is far from ideal but I think it might give reasonable results if built with a tight HF layout and decent components. At low frequencies the AC voltage at the base will be tiny, too small to measure but as the frequency increases the AC voltage at the base will begin to increase and I think the AC voltages at the base and collector will be the same at frequency Ft.

The best tool for measuring the voltage at the base and collector will be a sensitive RF probe with low capacitance. This could be a simple diode probe as long as it is sensitive enough to measure maybe 10mV pkpk at maybe 50-100MHz. A scope with a x10 scope probe will probably be (just) too deaf to see the signals. Alternatively a decent commercial RF millivoltmeter could be used here. Don't overdrive it with the sig gen or distortion will occur.


The Vce is set by the supply voltage V1 and the Ic is set by the negative voltage supply (V2) at the 1k emitter resistor. Because the emitter is fixed at -0.7V and the emitter resistor is 1k you need a -1.7V supply at V2 for 1mA Ic, -2.7V for 2mA Ic and -10.7V for 10mA Ic etc. So with this circuit you should be able to measure how Ft changes with Vce and Ic.

Hope this is OK, it's late and I'm a bit tired. I'll build and test this circuit tomorrow or over the weekend. I've posted it up in advance in case anyone can suggest improvements. It really is just a starting point for something a bit more elegant

Note that I don't have any design experience with Ge transistors so I don't know how much the 'leaky' nature of these devices will affect the results of the above test. But the circuit should be reasonable with silicon BJT devices?
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Regards, Jeremy G0HZU

Last edited by G0HZU_JMR; 9th Mar 2018 at 1:12 am.
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