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Low frequency characteristics revealed: How does the common gate amplifier affect signal gain?
In electronics, a common gate amplifier is one of three basic single-stage field-effect transistor (FET) amplifier topologies, typically used as a current buffer or voltage amplifier. In this circuit, the source of the transistor is the input, the drain is the output, and the gate is connected to a DC bias voltage (i.e., AC ground), hence the name. The corresponding bipolar junction transistor circuit is the common-base amplifier.
Application
While this configuration is used less frequently than a common source or source follower, it can be combined with a common source amplifier to create a Casco configuration. Particularly useful in CMOS radio receivers, especially near the frequency limits of FETs, as it is easier to impedance match and may have lower noise.
Low frequency characteristics
At low frequencies and small signal conditions, the circuit shown can be represented by a mixed π model. An overview of the amplifier's characteristics follows:
In general, the total voltage/current gain may be significantly less than the open/short circuit gain due to the effects of source and load resistance.
Closed loop voltage gain
Considering the input and output loads, the shutdown circuit voltage gain can be written as:
A_v ≈ \frac{g_m R_L}{1+g_m R_S}
This has a simple limiting form:
A_v = \frac{R_L}{R_S} \quad or \quad A_v = g_m R_L
It depends on whether g_m R_S is greater than or less than 1. In the first case, the circuit acts as a current follower, and the intermediate analysis is as follows: when R_S >> 1/g_m, the voltage source can be regarded as its Norton equivalent, and the usually output current is v_TH/R_S, and Norton The impedance is R_S. Due to the low input impedance of the amplifier, the driver delivers current v_TH/R_S to the amplifier through current sharing. The current gain is in units, the same as the load R_L, which produces an output voltage v_out = v_TH R_L / R_S according to Ohm's law, thus giving the first form of the above gain.
In the second case, R_S << 1/g_m, a Tefmeter representation of the source is useful, which yields a second form of gain, which is typical of voltage amplifier characteristics. Since the input impedance of common gate amplifiers is very low, Casco amplifiers are often used. Casco amplifiers place a common source amplifier between the voltage driver and the common gate circuit for voltage amplification using a driver with R_S >> 1/g_m.
Conclusion
Although common gate amplifiers are relatively uncommon, their advantages in certain situations cannot be underestimated. It not only provides good impedance matching, but also provides excellent low-noise performance in certain applications. Recognizing and using this type of amplifier is critical to understanding the full scope of modern electronic design. In this case, challenge yourself: How would you exploit the characteristics of a common gate amplifier to improve performance in future electronics projects?
