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Automatic gain control, known as AGC (Automatic Gain Control), is one of the basic functional blocks of any quality receiver. Its task is to maintain the output signal at an acceptable level regardless of the input signal fluctuations. In practice, this means that weak signals are amplified, while strong signals are automatically suppressed to avoid waking up the amplifier or unpleasantly "deafening" the operator.
In this article we will look at a simple and very practical solution for a low-frequency AGC that uses a commonly available BS170 type MOSFET transistor in combination with the popular LM386 audio amplifier. This is a circuit that has found its way into many QRP projects and home transceivers.
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Why low-frequency AGC?
AGC can be implemented at different levels of the signal path – either high frequency (HF), intermediate frequency (IF), or low frequency (LF). Each approach has its own advantages and disadvantages.
Low-frequency AGC, as in this case, is implemented after the detector, i.e. directly in the audio part of the receiver. The advantage is simplicity of connection and minimal intervention in high-frequency circuits. The disadvantage may be lower dynamics compared to VF AGC, but for many amateur applications this solution is fully sufficient.
Basic circuit: LM386 as an audio amplifier
The core of the circuit is the LM386 integrated circuit, which is extremely popular among radio amateurs. It offers sufficient power for small speakers or headphones, while requiring a minimum of external components.
In our case, however, the classic connection is expanded with a regulatory element provided by the AGC.
MOSFET BS170 as a controlled resistor
The key element of the AGC is the BS170 MOSFET transistor. This transistor is connected in such a way that it acts as a voltage-controlled resistor. This is a fundamental feature that allows for smooth gain control.
At low gate-source voltages (Vgs), the transistor is closed and its resistance is very high. However, as Vgs increases and exceeds the threshold voltage Vgs(th), the transistor begins to open and its resistance decreases.
In this connection, the BS170 is placed in the signal path before the amplifier input. This means that:
- with a weak signal, the transistor is closed and the signal passes without attenuation
- with a strong signal, the transistor opens and begins to attenuate the signal
The result is automatic maintenance of a constant output signal level.
Signal level detection
For AGC to work, it is necessary to obtain information about the output signal level. This is provided by a simple detection circuit consisting of a diode (for example, 1N4148), a capacitor, and a resistor.
The signal from the LM386 output (pin 5) is fed to a potentiometer that allows you to adjust the AGC sensitivity. From the potentiometer slider, the signal goes through a diode to the gate of the BS170 transistor.
The diode provides direct current rectification of the signal, creating a control voltage proportional to the amplitude of the audio signal. This voltage is then filtered by an RC element, which determines the AGC dynamics.
Low-frequency AGC adjustment: the role of the potentiometer
A very important element is the potentiometer in the feedback loop. Its upper end is connected to the LM386 output, the lower end to ground, and the runner leads to a diode.
This potentiometer allows:
- set the level at which AGC starts to operate
- affect the sensitivity of the regulation
- adapt the behavior of the circuit to specific conditions
Proper adjustment is crucial for optimal function – an overly sensitive AGC can “choke” even weak signals, while an insensitive AGC will not prevent the amplifier from waking up.
AGC time constants
The speed of the AGC response is determined by the values of the capacitor and resistor in the detection circuit, C8 and R10. These elements determine:
- attack time – how quickly the AGC responds to a strong signal
- decay time – how quickly the system returns to its original state
In practice, they are chosen so that the AGC responds quickly to sudden strong signals, but returns more slowly, thus preventing "pumping" of the volume.
Practical experience and characteristics
This type of AGC has been successfully used, for example, in QRP SSB transceivers. Measurements have shown that it can significantly limit the output level of strong signals, while weak signals remain clearly readable.
Typical behavior:
- without AGC the output can reach uncomfortably high levels
- with AGC the output is stabilized at an acceptable level
From a practical point of view, this means greater listening comfort and protection of the operator's hearing.
Influence of power supply and temperature
The circuit is sensitive to the supply voltage. When operating from a battery, voltage fluctuations can affect the AGC threshold levels. Therefore, it is advisable to use a voltage stabilizer, such as a classic linear regulator.
Another factor is temperature. MOSFET transistors have a temperature dependence, which is manifested by a change in the operating point. In practice, this means that:
- at higher temperatures the output signal may be lower
- at lower temperature it may be higher
However, these changes are usually within tolerable limits.
Possible modifications
Instead of the BS170 transistor, the 2N7000 type can also be used, which has similar characteristics. For higher power or specific applications, it is also possible to experiment with other MOSFET transistors.
Another option is to add a preamplifier in the receiving section, which compensates for any loss of sensitivity caused by AGC.
Conclusion
The low-frequency AGC with a MOSFET transistor is an elegant and easy-to-implement solution for amateur radio receivers. In combination with the LM386 circuit, it provides sufficient performance and listening comfort.
The main advantages are:
- simple construction
- parts availability
- good effectiveness in practice
For radio amateurs who like to experiment and build their own devices, this is an interesting project with real benefits. Anyone who would like to try a different connection can use AGC ND6T or AGC with A244D.