Signal Level Control (AGC)
Instructor:
Dr. George Collins
Presented By:
Rahul Chitturi
Contents
•Automatic Gain Control (AGC)
•Basic AGC Circuit
•Types of AGC
Reverse AGC
Forward AGC
•Various AGC Systems
Peak AGC System
Keyed AGC System
Delayed AGC System
•Noise Cancelation
Diode Noise Gate Circuit
Separate Noise Gate Amplifier
•Video Amplifiers
•Advantages of AGC
•Conclusion
Automatic Gain Control (AGC)
•AGC circuit varies the gain of a receiver according to the
strength of signal picked up by the antenna.
•AGC circuit produces AGC bias , which is a DC voltage
proportional to the input signal strength.
•DC voltage is obtained by rectifying the video signal as
available after the video detector.
•AGC bias is used to control the gain of RF and IF stages in
the receiver.
Basic AGC circuit
1. Unidirectional voltage is
developed across RL.
2. R1, c1 acts as AGC filter.
3. AGC bias voltages is fed
to the RF and IF amplifier.
4. Decoupling n/w – used to
isolate the amp stages from
common AGC line.
Types of AGC
•Reverse AGC – Shifting operating point towards cut off.
•Forward AGC - Shifting operating point towards saturation.
Reverse AGC
1. Reverse AGC requires
smaller changes in
voltage for full gain
control.
2. Voltage divider formed
by R1 and R2 provides
suitable fixed forward
bias
3. R3,C1 Decoupling nw
Forward AGC
1. For any increase in
signal strength, VBE must
increase to shift the
operating point towards
saturation.
2. R5 is used in series with
the collector circuit.
3. When AGC voltage
varies to increase Ic, Vce
decreases.
Various AGC systems
•Peak AGC system / Non keyed AGC system
•Keyed AGC system
•Delayed AGC system
Peak AGC system
This system is based on sampling the
synch tip level.
1. During synch pulses of the modulated
signal, D2 conducts & C1 charges.
2. Voltage across R1is then filtered using
R2,C2 & produces AGC voltage.
3. During periods other than synch pulse,
diode is under RB condition.
4. But the C1 discharges through
secondary winding of the transformer.
Keyed AGC system / Gated AGC
•In this, AGC rectifier is allowed to conduct only during horizontal
synch pulses
•This is done with the help of fly back pulses derived from the
output of horizontal deflection circuit of the receiver.
Delayed AGC
•AGC voltage is proportional to signal strength.
•Even weak RF signals will produce some control voltage.
•This when applied to RF amplifier will tend to reduce its
gain.
•Hence, RF amplifier is not fed any AGC voltage till the
signal strength attains a predetermined level.
Noise Cancellation
The circuit which is used to cancel noise in AGC
and synch separator circuits is called Noise
cancellation circuit.
Method 1 – Diode noise gate
circuit
1. D2 is used as a switch which opens in the
presence of noise.
2. R3 is used to provide forward bias to the diode
so that it pass noise free signals to video amplifier.
3. For strong noise signal D2 is RB, thereby
stopping noise pulses from reaching the amplifier.
Method 2 – Separate noise gate amplifier
1. The output of video amplifier is
the inverted composite video
signal.
2. Noise gate is a grounded base
amplifier. It amplifies the noise
pulse without inversion of its
polarity.
3. The output of video amplifier &
Noise gate is added.
4. Thus noise pulse is cancelled on
addition.
Video Amplifiers
It is the one which is used to amplify the video signal. An
inverting amplifier is used generally, hence inverted composite
video signal is obtained as the output and fed to the cathode of
the picture tube.
RC Coupled Amplifier
High frequency compensation
To compensate the fall in gain at high frequencies, peaking
coils in different configurations are used.
Methods of using peaking coils
•Shunt Peaking
•Series Peaking
•Combination Peaking
Advantages of AGC
•Maintaining constant contrast and intensity of the picture.
•Constant contrast when receiver is switched from one station
to another.
•Reducing Amplitude and cross modulation distortion.
•Increasing gain of weak signals.
•Reducing flutter in the picture due to passing aero planes and
other fading effects.
•Maintaining constant sound signal.
•Easy separation of synch pulses.
Conclusion
In this presentation we have discussed about the Automatic Gain
Control, the basic circuits used for the AGC and its different
types.
We also discussed about the different systems in which these
AGC systems are used and the method in which they are used.
References
•http://en.wikipedia.org/wiki/Automatic_gain_control
•http://www.eecg.utoronto.ca/~kphang/papers/2001/martin_A
GC.pdf
•http://v2.embedded.com/design/214600265
•http://www.qsl.net/va3iul/Files/Automatic_Gain_Control.pdf
•http://www.tpub.com/neets/book8/31c.htm
Thank you