Boosting Performance of RF Amplifiers with Parallel Tuned Circuit Design

Here it is described how to design a typical RF amplifier stage with parallel tuned circuit for boosting RF amplifier performance. It describes the configuration of the amplifier and its differences from a typical common emitter amplifie circuit. It is explained how the use of a parallel tuned circuit as a collector load and the tapping of the collector of transformer into the tuned circuit can improve matching and prevent damping, which would reduce the Q-factor and selectivity.

It is also explained how one can take the output from a secondary winding on the main inductor and how it is possible to improve selectivity by using multiple tuned circuits to form a bandpass filter. The importance of optimal coupling between the tuned circuits for the proper functioning of the filter is also discussed, with the consequences of too loose or too tight coupling explained.

The circuit diagram of a typical RF amplifier stage is presented below. 

circuit diagram of RF amplifier with tuned circuit
This circuit can also be used as an IF amplifier in a superheterodyne receiver and is bit different from the common emitter BJT amplifier circuit(see 2n3904 amplifier). The amplifier operates in Class A and employs a small-signal NPN transistor in a common-emitter configuration.

The key difference between above RF amplfier circuits above and typical common emitter amplifier lies in the use of a parallel tuned circuit as the collector load in the RF amplifier. To enhance matching and prevent damping, which would reduce the Q-factor and selectivity of the tuned circuit, the collector of transformer TR1 is tapped into the tuned circuit instead of being directly connected to it. The maximum impedance of the tuned circuit is at resonance, resulting in maximum gain at the resonant frequency. By using a high-Q factor tuned circuit, the amplifier's response can be limited to a narrow range of frequencies.

The output to the next stage is taken from a secondary winding, L2, on the main inductor, L1. To further improve selectivity, multiple tuned circuits can be used to form an effective bandpass filter, as shown in circuit diagram below. 

tuned bandpass filter circuit diagram
Such circuit is also called tuned bandpass filter. 

When constructing an RF filter with multiple tuned circuits, the optimum coupling must be achieved between them, as shown in the frequency response for two coupled tuned circuits figure below.

Frequency response for two coupled tuned circuits

If the coupling between the two tuned circuits is too loose, the frequency response will become flat and inadequate output will result. On the other hand, if the coupling is too tight, the response will broaden and exhibit a double-hump shape. The optimal coupling, also known as critical coupling, yields a frequency response with a flat top and steep sides.

In this way we can realize an RF amplifier with parallel tuned circuit design. In the RF amplifier circuit diagram, the resistor R1, R2,R3 and the capacitors C1, C3 are biasing resistors and coupling and bypass capacitors. The tutorial BJT transistor biasing explains with example calculation how to calculate the value of these biasing resistor and capacitors. Or to calculate these biasing and coupling, bypass capacitor the online BJT amplifier calculator can be used.


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