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Last updated on 2024年10月18日

How do you design a transistor amplifier with a specific gain and input/output impedance?

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Transistor amplifiers are essential components of many electronic circuits, such as radios, sensors, and microcontrollers. They can amplify weak signals, such as audio or radio waves, and control the current or voltage of other devices. However, designing a transistor amplifier with a specific gain and input/output impedance requires some careful calculations and testing. In this article, you will learn how to do that using a common-emitter configuration and some basic electrical testing tools.

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Mihajlo Petkovic

Currently working on developing raspberry-noaa-v2 project on Github for receiving satellite images from the weather…
Pinal Prajapati

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1 Choose a transistor type

The first step is to choose a suitable transistor type for your amplifier. There are two main types of transistors: bipolar junction transistors (BJTs) and field-effect transistors (FETs). Both have three terminals: the base (B), the collector (C), and the emitter (E) for BJTs, and the gate (G), the drain (D), and the source (S) for FETs. The main difference is that BJTs are current-controlled devices, while FETs are voltage-controlled devices. This means that the current flowing from the base to the emitter of a BJT controls the current flowing from the collector to the emitter, while the voltage applied to the gate of a FET controls the current flowing from the drain to the source. Depending on your application, you may prefer one type over the other, based on factors such as power consumption, noise, frequency response, and cost.

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Pinal Prajapati

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Choose the correct transistor type and biasing method. This will help you create a transistor amplifier with a specific gain and input/output impedance. Calculate resistor values for biasing and feedback networks to reach the desired gain and impedance. Ensure proper matching of input and output impedance with source and load. Transistor datasheets help calculate amplifier configurations. Examples include a common emitter and a common collector. Test and adjust circuit parameters for optimal performance.

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2 Calculate the biasing resistors

The next step is to calculate the values of the biasing resistors that will set the operating point of your transistor. The operating point, also known as the quiescent point or Q-point, is the combination of DC voltage and current that determines the linear region of operation of your transistor. This is important because you want your amplifier to operate in the linear region, where the output signal is proportional to the input signal, and avoid distortion or saturation. To calculate the biasing resistors, you need to know the desired output voltage (Vout), the supply voltage (Vcc), and the DC current gain (beta) of your transistor. You can find these values in the datasheet of your transistor or measure them with a multimeter. Then, you can use the following formulas to calculate the base resistor (Rb), the emitter resistor (Re), and the collector resistor (Rc):

Rb = (Vcc - Vout) / (beta * Vout / Re)

Re = Vout / Ie

Rc = (Vcc - Vout) / Ic

where Ie and Ic are the emitter and collector currents, respectively, and are equal to Ie = Ic = beta * Ib, where Ib is the base current.

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Mihajlo Petkovic

Currently working on developing raspberry-noaa-v2 project on Github for receiving satellite images from the weather satellites on VHF while studying electronics engineering. :)
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The formula for calculating Re is incorrect. It isn't Vout/Ie, rather Ve/Ie. Output is taken from the collector, not the emitter in a common emitter configuration. Therefore, an appropriate value of emitter degeneration voltage Ve should be taken as a starting design criterion and used to calculate emitter resistor. Rb also should Ve instead of Vout in the formula. For maximum voltage swing, the transistor should be biased to produce Vc=(Vcc-Ve-Vce_sat)/2 in static mode.

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3 Calculate the input and output capacitors

The third step is to calculate the values of the input and output capacitors that will block the DC component of your signal and allow only the AC component to pass through. The input capacitor (Ci) connects the input signal to the base of your transistor, while the output capacitor (Co) connects the collector of your transistor to the load. The values of these capacitors depend on the input and output impedance of your amplifier, as well as the frequency range of your signal. You can use the following formulas to calculate the input and output capacitors:

Ci = 1 / (2 * pi * f * Rin)

Co = 1 / (2 * pi * f * Rout)

where f is the lowest frequency of your signal, Rin is the input impedance of your amplifier, and Rout is the output impedance of your amplifier. The input impedance of your amplifier is equal to Rb in parallel with beta * Re, while the output impedance of your amplifier is equal to Rc.

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4 Calculate the voltage gain

The fourth step is to calculate the voltage gain of your amplifier, which is the ratio of the output voltage to the input voltage. The voltage gain of a common-emitter amplifier is given by the following formula:

Av = -Rc / Re

The negative sign indicates that the output signal is inverted with respect to the input signal. This means that when the input signal goes up, the output signal goes down, and vice versa. The magnitude of the voltage gain depends on the values of Rc and Re, which you have already calculated in the previous steps.

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5 Build and test your amplifier

The final step is to build and test your amplifier using a breadboard, a power supply, a signal generator, and an oscilloscope. To assemble and test your amplifier, start by connecting Vcc to one end of Rc and to one end of Rb. The other end of Rc should be connected to one end of Co and the collector of your transistor. Co should be connected to the load, such as a speaker or headphone. Re should be connected to the emitter of your transistor and the other end to ground. Ci should be connected to the input signal and the other end to the base of your transistor. The ground of the input signal should be connected to the ground of the power supply. Turn on the power supply and signal generator, and set them to desired values. Connect the probes of the oscilloscope to the input and output terminals of your amplifier, and observe waveforms on screen. Adjust amplitude and frequency of input signal, and measure output voltage and voltage gain of your amplifier. Compare results with calculations, checking for any errors or discrepancies.

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6 Here’s what else to consider

This is a space to share examples, stories, or insights that don’t fit into any of the previous sections. What else would you like to add?

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Electrical Testing

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How do you design a transistor amplifier with a specific gain and input/output impedance?

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1 Choose a transistor type

2 Calculate the biasing resistors

3 Calculate the input and output capacitors

4 Calculate the voltage gain

5 Build and test your amplifier

6 Here’s what else to consider

Electrical Testing

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