Calculations and Simulations with Pads and Feedback Amplifiers
Adding a Pad
We now adopt the often seen topology of an amplifier followed by a pad. The circuit with a 6 dB pad (R12, R13, and R14) is shown below:
Gain (S21) at 10 MHz, which is within the filter passband, is down to 14.7 dB. A sweep of input S11 is shown below:
The input match has the narrow band characteristics of the output filter. But they are modified. The in-band match at 10 MHz is a return loss of 19.8 dB. The out of band match is 11.8 dB return loss or better for all frequencies. This is less than a 2:1 VSWR and is good enough to terminate most of the out of band components from a diode ring mixer that cause severe IMD. There is more to be gained with even better terminations, but this represents a first step.
Removing Amplifier Feedback
Finally, we ask what would happen if we modified the amplifier to remove the parallel feedback. This is shown in the following :
In this circuit, we have moved the connection of the bias and feedback network (R3, R4, C5) from the collector to the bypassed and decoupled power supply point. This leaves the DC biasing unchanged, retaining an emitter current of 19 mA. The emitter degeneration is left unchanged. The result is an amplifier with a 10 MHz gain of almost 21 dB, even with the 6 dB pad in place. The input match is shown from 5 to 15 MHz in the following plot:
We see a smooth downward trend in S11, modified by the output tuned filter. At 10 MHz the input impedance is 84.5 – j85.3, which has the equivalent of a 171 Ohms in parallel with 94 pF. (S11= -2.38 dB at -35.6 degrees.)
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