Wideband ALC amplifier circuit and working principle

Function of the circuit

This is a circuit that stabilizes an unstable input signal at a certain level. It is used in circuits with high performance requirements. In these signal generators, the output level will fluctuate due to the uneven frequency characteristics. If this circuit is added, it can be automatically controlled to keep the signal at a certain amplitude. In addition, in order to reduce the output impedance, the circuit adds a push-pull stage.

How the Circuit Works

This circuit uses a broadband amplifier IC whose amplification factor can be controlled by an external voltage, thus having a compression characteristic of 20DB.

In the input circuit, the resistor marked with ★ is added to reduce the input level. When driving a 50-ohm load, it is difficult for TL026 to obtain a large output amplitude, so a push-pull buffer amplifier composed of transistors is added to the circuit to reduce the burden on TL026.

The output of TL026 is differential. If the load resistance is not equal, the frequency characteristics will change, so C2 and R4 are connected to the leads. The potential difference between leads 2.7 can control the amplification factor. Because of DC drift, OP amplifier A8 is used. Diode D1 rectifies the output and compares it with the reference voltage. Diode D2 is added to compensate for the temperature characteristics of D1.

OP amplifier A2 acts as a comparison circuit. When the output level increases, the current flowing through D1 will increase. A2 integrates it and outputs a negative voltage, which is added to the inverting input of A3, increasing the potential of lead 2 of A2 relative to lead 7, thereby reducing the amplification factor of A1.

Component Selection

Because the whole circuit forms an ALC loop, it is relatively easy to select components, but the stability of the diode D5, variable resistor VR1, and resistors R12 and R13 that generate the reference voltage is a key consideration when selecting components. In order to make the forward voltages of diodes D1 and D2 equal, thermal coupling should be used. The rectification characteristics of ordinary small signal switching diodes will decrease at 50MHZ, so Schottky diodes should be selected.

Adjustment and electrical characteristics

Without adding the resistor with a ★ mark, input a signal of -20DBM, F=1MHZ, adjust VR1, and obtain a voltage of 1VP-P at the output. Then amplify the input voltage by 10DB to verify whether the output changes. Photo A shows the frequency characteristics when the input level is -20DBM, F=100KHZ~100MHZ. Because the division is 20DB/DIV, it is 50MHZ/-3DB.