automatic gain control amplifier - Page 1
Forum section: electronics / projects

author: electros6
author_rank: Regular Contributor
timestamp: September 06, 2014, 04:35:07 am
content: Hai guys ,I tried to design an automatic gain control amplifier over the bandwidth of DC to 50 MHz Please share any design ideas to design my amplifier using transistors also please provide any useful links about this topic

author: PChi
author_rank: Frequent Contributor
timestamp: September 06, 2014, 04:54:00 pm
content: Is the Linear Technolgy LT1228 suitable?There are also analog multipliers available from Analog Devices.

author: Richard Head
author_rank: Frequent Contributor
timestamp: September 07, 2014, 10:02:16 am
content: If it's for an IF amplifier application you can try an MC1350 with a separate op-amp and output power sensor wired in a closed loop configuration. I'm not sure what your power level is but the MC1350 can do about +10dBm I think before compression becomes a problem.The MC1350 is practically obsolete now so this choice would not be a good idea for new production but only hobby/experimentation purposes.

author: David Hess
author_rank: Super Contributor
timestamp: September 07, 2014, 09:06:33 pm
content: Tektronix used "paraphase amplifiers" for wideband gain controls but they amounted to Gilbert cell multipliers like you would find in an active mixer. Such could certainly be build using discrete but matched transistors.Gain can be varied continuously from -1 to +1 without significantly affecting bandwidth or transient response.A simpler alternative would be an operational transconductance amplifier like the LT1228 which PChi mentioned.

author: salbayeng
author_rank: Frequent Contributor
timestamp: September 08, 2014, 07:31:20 am
content: Hi , Can you re-define the frequency range, specifically the low end.DC doesn't really have an "amplitude" that can be measured in order to control it.And what kind of amplitude? volts, millivolts .Also what kind of attack and decay timing do you need.Is it a complex waveform that needs to be preserved or would a simple log amp be sufficient?

author: electros6
author_rank: Regular Contributor
timestamp: September 09, 2014, 02:51:38 pm
content: I actually tried to used the AGC amplifier in my frequency counter front end.

author: David Hess
author_rank: Super Contributor
timestamp: September 09, 2014, 07:43:00 pm
content: Most RF frequency counters have limiting amplifiers instead of AGC amplifiers. Universal counters do not use AGC amplifiers because they operate down to DC.

author: electros6
author_rank: Regular Contributor
timestamp: September 10, 2014, 03:04:12 am
content: I have to amplify the clamp voltage of the protection diode at the input stage say 600mv P-P , at the same time if I fed 200mv P-P I have both the signal to be in the range of my Schmitt trigger

author: PChi
author_rank: Frequent Contributor
timestamp: September 10, 2014, 05:48:20 pm
content: Rather than use an AGC which might struggle with controlling the gain over such a wide bandwidth it might be easier to use an analogue comparator and vary the threshold to alter the sensitivity from 200 mV to 600 mV.To cover a wider input voltage range would a Programmable Gain Amplifier (PGA) be useful? Possibly something like the Texas Instruments PGA870.

author: David Hess
author_rank: Super Contributor
timestamp: September 10, 2014, 06:45:58 pm
content: Using a limiting amplifier is the simplest solution. They are designed to saturate without extending recovery time so they may be overdriven without problems. Comparators by themselves function this way.An alternative would be a comparator or Schmitt tigger (most comparators function as Schmitt triggers anyway to a minimum extent) with an automatically adjustable threshold based on the peak to peak input signal. Some oscilloscope trigger circuits can work this way.http://www.linear.com/docs/4390

author: salbayeng
author_rank: Frequent Contributor
timestamp: September 19, 2014, 10:15:43 am
content: One can get a limiting amp by using part of a FM discriminator chip , if memory serves me correctly the NE526 ? or SA526 . n.b. Pretty much all limiting amps need to be AC coupled.The other thing you might be thinking of is known as a data slicer (used for example on UHF remote control receivers) , you take the signal which is an unknown DC voltage with a variable amplitude "data" on top of that , the signal is fed directly into the positive terminal of a comparator, and the same signal through a lowpass series R and a shunt C to the negative input , so the reference voltage (at the neg input) is just the slow average of the signal voltage, and the output is high or low depending if the signal is above or below the average value. (this is also how AC trigger is implemented on some scopes)You can combine the above concepts using a bunch of cascaded (unbuffered) 74C04 (CD4009) hex inverters , AC couple in using a large capacitor, and connect a large resistor from in to out of the first stage (or from in to out of the third stage).If you want to scratch build with discrete transistors, then just cascade "inverters" made from a complimentary npn/pnp common emitter stage (with schottky miller clamps)