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The circuit has a lower cut-off frequency near 150 Khz and the higher cut-off is limited to the inter-junction capacitances of the transistor. You can definitely use this pre-amp at 49Mhz. If not try using a more higher frequency transistor.


Well-Known Member
Actually, since the output resistance of this preamp is about 1k, a 3.25 pf load would yield a -3dB BW of 49 MHz, even assuming the preamp has infinite bandwidth, which it doesn't. Also, due to feedback, the resistance at the base is only about 25 ohms, which means that, in most applications, the resistance of the source will significantly affect the lower -3dB point, the low-frequency gain and the upper cutoff frequency. With a zero impedance source, the lower corner is at about 6 MHz. With a 100 ohm source, the low freq corner will be at about 1.2 MHz, but the gain will be lower. BTW, I didn't calculate all these values. I ran some Spice simulations. I couldn't find a model for BFY90, so I used MPS5179. A higher Ft transistor would help some, but I think the 1k output resistance is a significant limitation. There are probably some good preamp schematics available for 49 MHz, but I don't think this is one of them.
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