The AGC is used in many system especially communications. This AGC  works at 50Mhz, and the output voltage is set by a digital signal that is needed by Self-calibrating systems such as automatic test equipment which requires the high frequency response. During calibration, the AGC output voltage will be calculated by test equipment to get accuracy requirements. The AGC output will be increased until the system is within specifications.

RF AGC with Digital Control circuit schematic

This is a RF AGC with Digital Control circuit. This circuit uses The HI5731 as DAC which is used to accomplish the digital control. Besides that the DAC is the reference input for the AGC circuit, and the AGC output voltage is set by the DAC. The HI5731 has some advantages, it is cheap, it can be update at a 100MHz rate, it needs 5V supplies and the output interfaces well with the AGC circuit. This circuit uses a three transistor to make variable gain amplifier which is the heart of this circuit.

The transistors is set up to long-tailed pair configuration. The emitter current of the long-tailed pair changes, when we vary the base voltage of Q3. It will force gain change according to the
following equation :

G = K*VIN *VB3,

where VB3 is the base voltage of Q3 and K is a function of the emitter current. The transistor matching will determine bias-stability and The gain-control parameters of the circuit. -0.04 to -4.1V is the usable voltage range of VB3. So we have, gain range of 0.8 to 17.6dB if we use Vin=100mV. This circuit has gain span of 16.8dB. R4 is used to control the gain, the gain will increased when R4 is increased. However, while the frequency response decreases, the gain span stays constant at approximately 16.8dB. [Source: Intersil Application Note]