In order to make a high-frequency amplifier, the bipolar transistor used in the common emitter configuration is sometimes defined by its small signal characteristics assuming that the collector emitter voltage (VCE) and collector current (IC) of the gain transistor are specific values. Therefore, it is necessary to produce these two conditions at the same time and have good accuracy when designing the circuit, which may be a challenge to the traditional bias circuit.

Design and application of active bias control

Figure 1: conventional bias circuit

In the traditional circuit, in order to reliably set the collector emitter voltage (VCE) and collector current (IC) of the transistor to the values we want, a resistor needs to be connected in series with the emitter and grounded, and then considering the signal gain, we may also need to place a bypass capacitor at both ends of the resistor in parallel.

With or without bypass capacitance, the emitter may not be really in signal ground because both elements exhibit impedance. In the expected operating frequency range, there may be self resonance at some frequency points. RF transistor has better small signal performance because its emitter is really grounded. Due to the “disturbance” of the emitter, RF transistors may sometimes exhibit completely different small signal characteristics, as shown in Figure 1.

One way to solve this problem is to use active bias control, as shown in Figure 2.


Figure 2: active bias circuit.

Using the active bias, the collector emitter voltage (VCE) of the gain transistor is equal to the rail voltage minus the base emitter voltage (VBE), and the IC of the gain transistor is almost equal to the result obtained by dividing VBE by the shunt resistance R1, as shown in Fig. 2.

Now, the emitter of the gain transistor can be grounded correctly according to the requirements defined by the small signal parameters, and the bias conditions are predictable and stable.

SPICE simulation of active bias configuration is shown in Figure 3.

Figure 3: SPICE simulation of active bias

Pn2222 is not a true UHF Transistor, but it can be used in my simulator. The figure shows this well. If your spice simulator has a really fast RF transistor model, it should be very easy to use.

Figure 4: grounded emitter with source bias control.

Responsible editor: GT

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