AGC ND6T
What is AGC?
AGC (Automatic Gain Control) is an important part of any device. The dynamics of the signals coming from the antenna is so great, that reception without this circuit is very unpleasant. Constructing a quality AGC is quite difficult, but even a simpler circuit can help significantly.
In the article you will read What is AGC (TCA440)
I have used my own circuit solution several times A244D (TCA440). The principle is simple: We direct the NF signal from the volume regulator and feed it to the pin 10 tohto I.. The range of regulation is possible up to 60 dB. However, there is a downside, that the MF part of the A244D works only at low frequencies. Thus, this type of AGC would not prevent the RF circuits of the receiver from waking up.
AGC ND6T
ND6T developed an AGC circuit for the QRP TCVR uBITX, which is connected to the input of the receiver. In principle, the resistance of a pair of MOSFET transistors to ground is changed using voltage.
The circuit consists of an amplifier with transistor Q1 behind which is connected a rectifier with diodes D1 and D2. If the NF signal supplied to this amplifier is less than 30 mV, so its output voltage is relatively small and will not change the quiescent setting of MOSFETs Q2 (In a series) to Q3 (parallel). Only a minor insertion loss of approx. 0,6 dB.
If we introduce a signal with a higher level, the voltage behind the rectifier will already start to rise and the operating points of Q2 and Q3 will change. An NF signal with a level of 1V would represent about 3V of DC voltage. This will create attenuation 34 to 50 dB (by frequency). The AGC time constant is determined by the capacitance C4 and resistance R6, so 1 second.
I made the connection on a piece of universal printed circuit board. Attention must be paid to the principles of HF construction, in order not to reduce the possible attenuation or influence the AGC by other HF voltage. A shorter time constant worked for me, and significantly shorter (150k is 1uF).
I would recommend the AGC ND6T as a suitable addition for HAM constructions of receivers and TCVRs.
