VCA821: Automatic Gain amplifier frequency Response.

Hello Nick,
Thank you for your input.

As per your suggestion I have done transient analysis, but I see for 1.5V and 2V input the control voltage from OPA820 to VCA to be same. Attached the transient results for both the inputs. Is this correct? does control voltage always be same for AGC?

Also, can I analyze AGC performance with input and output gain comparison instead of voltage so that I can be clear as the name says " gain control" is maintained.

Hi Harshini,

For AGC applications, the control voltage will change in relation to the input signal of the AGC circuitry. The waveforms will look something like this:

The attack time and decay time determine how fast the AGC circuitry responds with a variable gain increase/decrease in relation to the input signal level. If you increase the amount of time the simulation runs for (to say 5ms) you will see the gain control voltage settle to a value and the output will settle to a set value that is not saturating the output of the VCA821 like you are currently seeing. 

These two E2E threads explain this well:
https://e2e.ti.com/support/amplifiers-group/amplifiers/f/amplifiers-forum/1106330/vca810-vca810 

https://e2e.ti.com/support/amplifiers-group/amplifiers/f/amplifiers-forum/935654/vca821-i-think-there-is-something-wrong-in-the-picture-of-datasheet-is-there-someone-who-have-succeed-simulating-the-circuit-of-this-picture 

I believe that because you have such a large input signal you are seeing issues like discussed in the first thread.

Thanks,

Nick

Hello Nick,

The two threads were bit helpful thanks for sharing!

My application for this AGC is powerline communication which operates between 1 to 30MHz BW, so within this freq I want to maintain the gain control. 

As Kai suggested in the 1st thread I have cascaded VCA820 to obtain this high BW.

These are the few questions that I am not able to clear out

1. According to my application AGC should work between 30ns to 200ns the below circuit is working from 1ms, please suggest me the changes that are needed.

2. Is transient analysis only way I can determine AGC functionality or is there any way through AC analysis I can check if AGC is working ?

3. If transient is the only way, how to give varying input signal voltage like you have shown in the above image(how to suddenly increase input voltage and check output clamp).

4010.AGC simulation (1).TSC

Thank you,

Harshini

Hi Harshini,

I am still working to understand why the below circuit is taking 1ms for the control voltage to settle. 

To answer your question about varying the input signal voltage:

There is in fact a way in TINA-TI to vary the input voltage signal. Through the use of voltage controlled switches, you can have to different sine waves and switch one on and the other off at a set time to simulate a change in input voltage. I have shown this below. I have also attached your schematic file that has added this change.

4336.AGC simulation (1).TSC

Thanks,

Nick

Hello  Nick,
The voltage controlling switches worked, thanks for the input!

I just have one final question, what are the changes needed in the circuit (assuming R1, R2, R12 and C1) for the voltage clamping to start at 30ns?

Thank you,

Harshini

Hi Harshini,

Combining threads (other linked here)

I am looking into what is causing the voltage clamping to take longer than 30ns. It is possible that the integrator amplifier is being bandwidth limited. I will provide you an update by the end of the week with what I find.

Thanks,

Nick

Thank you for your support Nick! 
Will be awaiting for your answer.

Thank you for your patience! I should have more information by the end of day tomorrow.

Thanks,

Nick

Hello Nick,
Any update on the circuit? With the circuit below the AGC action is starting at 200ns, but could you help me to settle it down for around 250 or 300ns?

AGC for 200ns.TSC

Thank you,

Harshini