Single-Stage, Small-Signal RF Amplifier Designs

Key Takeaways

• What small-signal RF amplifiers do.

• Voltage divider transistor biasing for small-signal RF amplifiers.

• Single-stage, small-signal RF amplifier design steps.

Almost all electronic circuits rely on amplifiers. These amplifier circuits enlarge the input signals they receive. A basic amplifier circuit consists of bipolar junction transistors, which are biased such that the device operates in the active region. The active region of transistors is used for amplifying purposes. When the transistor is biased to be in the active region, an input signal applied across the input terminals fluctuates the output current. The fluctuating output current flowing through the output resistor produces amplified output voltage. 

Amplifiers that enlarge weak RF input signals with small fluctuations in the output current compared to the quiescent point are called small-signal RF amplifiers. Small-signal RF amplifier designs can be common-base, common-emitter, or common-collector configurations. In this article, we will focus on common-base, small-signal RF amplifier designs.  

Single-Stage, Small-Signal RF Amplifiers 

A small-signal RF amplifier can be either a single-stage or multi-stage amplifier. In a single-stage, small-signal RF amplifier, the input is amplified using the transistor. The transistor biased in the active region is supplied with an input signal. According to the variation in the input signal, the output current varies, and the amplified output voltage is obtained. A small-signal RF amplifier can also be called a voltage amplifier.

The inputs required for designing small-signal RF amplifiers are the output current, output voltage, DC biasing voltage, and the transistor current gain . The current gain is an internal characteristic of a transistor, obtained from the datasheet.

Next, we will look at transistor biasing for small signal RF amplifiers.

Voltage Divider Transistor Biasing  for Small-Signal RF Amplifiers

The design of small-signal RF amplifiers begins with a transistor biasing circuit. Let’s use a common-base, small-signal RF amplifier design with an NPN transistor with a given high value as an example. The collector current and voltage across the collector output resistor form the output current and output voltage, respectively. Let the input bias voltage be VCC. Note that in this example, we are using a voltage divider transistor biasing circuit for further discussion, as it is the most commonly used transistor biasing for amplifiers and is a simple circuit with good operating point stabilization.

When the transistor is in a common-base configuration, resistor R2 and RC from the base and collector terminals of the transistor connect it to the DC input voltage VCC.  The resistor R1  and RE connect the base and emitter terminals of the transistor to the ground, respectively. By suitably selecting the resistor values, the operating point of the transistor amplifier is fixed. The voltage divider transistor biasing makes the operating point of the transistor almost independent of . That's why voltage divider biasing is also called biasing circuit independent of beta.

To design this type of transistor, there are a few key steps that should be followed.  

Single-Stage, Small-Signal RF Amplifier Design Steps

The following steps should be followed when designing small-signal RF amplifiers with voltage divider transistor biasing:

1. For the given design parameters, output current (I_C), output voltage (V_C), DC biasing voltage (V_CC), and the transistor current gain , choose the transistor operating point 

2. Considering bias stability, the voltage across emitter resistor R_E is fixed as V_E

3. For high beta transistors, the collector (I_C) and emitter(I_E) currents are approximately equal. With knowledge of I_E and  V_E , the resistor  R_E is designed using the equation .

4. The resistor RC is calculated from the equation

5. From I_C and , the base current is .

6. For silicon transistors, the voltage drop across the base and emitter V_BE is equal to 0.7 V. The base voltage V_BB can be calculated as .

7. The current from the DC input voltage to the base terminal of the transistor is I_BB. Assuming the I_BB value, the resistor is .

8. By following the equation R , resistor R₂  is designed 

9. The coupling capacitors C_in and C_C are designed such that they block DC and only allow AC to pass through them.

10. The bypass capacitor C_E is designed as per the equation .

By following the above steps, a small-signal RF amplifier can be designed. In small-signal RF amplifier designs, the stabilization of the operating point of the transistor is an important concern. The internal characteristics of the transistors vary with temperature, and it can influence the operating point and the functioning of the amplifier. Cadence’s suite of software can assist you in designing small-signal RF amplifiers and stabilizing their operating points. 

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