# How to Bais a tuned rf amplifier and compute I/O Impedances

Can anyone point me in the right direction on how to implemented the depicted tuned r.f amplifier? Since the load resistance at the collector is replaced by the LC tank how does one go about in calculating the values to bias the circuit? Would the biasing be affected if the LC tank is tunable to adjust the desired frequency of resonant? {ganged with the first LC tank} Also how does one go about in designing/finding the input/output impedance. I know that at resonant frequency the LC tank has a high impedance and elsewhere low, is this information useful in finding the impedance?

• The bias voltage depends on Vcc being stable, such as 5 volts or 12 volts. Then some rational values about R1, R2 and R3 can be established.
– user105652
Mar 3 '18 at 3:20
• @Sparky256 In other words, I can treat L1 and C4 as a wire for biasing the transistor ? Mar 3 '18 at 3:28
• L1 and C4 form a resonate circuit, tuned to the frequency desired, and have little effect on DC bias. The bias for Q1 is controlled by ratio of (R1/R2)-.65 volts across R3.
– user105652
Mar 3 '18 at 3:35
• Get a simulator and use it to do all the heavy work. I'm serious about this. Mar 3 '18 at 9:25
• There is a feedback path ----- the Cob, or the capacitance from base to collector ---- that is a problem. Its also called the Miller Capacitance, and Cbase will be huge compared to Ccollector. Read up on Miller Effect. Mar 3 '18 at 17:40

The AC equivalent circuit would look like this, assuming that C1, C2, C3 have very low impedance (large capacitance values) at the frequency of interest:

simulate this circuit – Schematic created using CircuitLab
There is a potential problem with this circuit. When Lin combined with Cin has a resonant frequency very close to the resonant frequency of Cout combined with Lout, it is a potential oscillator. Because Q1 has significant gain, its collector-to-base capacitance feeds a portion of the output signal back to the input: oscillations result.

R1 and R2 could be made small enough that input impedance is reduced to the point where oscillations don't occur. Or you could bias Q1 so that its input impedance is low. Similarly, the output impedance can be reduced by loading the tank with resistance. Such techniques are often used to make potentially-unstable amplifiers refrain from oscillation.
At much higher frequencies, gain of the transistor is so low, that potential oscillations are not a problem.

• Should I be fine if the intended range of the LC of used is vhf 88-108Mhz? Mar 3 '18 at 4:43
• Not really suitable method for an FM radio without an IF because it is hard to tune two ganged high Q filters Mar 3 '18 at 6:10
• The miller capacitance will produce a 90 degree phase shift back across the input LC so it won't oscillate on that basis. Inductor coupling is another thing of course. Mar 3 '18 at 9:23
• @learnmore Many high-frequency transistors are potentially unstable at VHF frequencies. High-Q tank resonators (input side & output side at the same frequency) matched to the transistor's input impedance & output impedance can easily lead to the circuit being an oscillator instead of amplifier. Mar 3 '18 at 14:12
• @Andyaka You bring up another aspect of stabilizing this circuit: neutralization...brainkart.com/article/Neutralization-Methods_12559 One method actually adds some inductive feedback to resonate-out the transistors' troublesome collector-to-base capacitance. Mar 3 '18 at 15:55