Differential amplifier gain

Status
Not open for further replies.
S

Sambhav_1

Full Member level 2
Joined
Jul 13, 2018
Messages
125
Helped
1
Reputation
2
Reaction score
3
Trophy points
18
Activity points
941
Hi,

i am lookin for designing 100MHz bandwith diff. amp but I am getting this kind of gain plot. I want to know why is this peak coming and how can i eliminate?


 
Last edited:

Sort by date Sort by votes
You apply the same signal on both inputs. What you see is your common mode gain.
 

Upvote 0 Downvote
The differential input signal is the difference between the two inputs which in your case is 0. You only change the common mode level of the 0 amplitude input signal.

Set two voltage sources to, for example, 0.5 V AC amplitude and whatever DC level you have. One should have 0 degrees phase and the other 180 degrees.

This means that your input signal will be 0.5-(-0.5)=1 V in amplitude.
 

Upvote 0 Downvote
You don't show which output signal is used to plot the gain, we can just guess that it's a single ended output, resulting in relative large common mode gain. You should use output voltage difference instead.
 

Upvote 0 Downvote
with this, how can i check my differential gain? i need 15dB differential gain. so should i check like this, amplitude at output/amplitude at input and converting it into dB.
 

Upvote 0 Downvote
HI,

differential_gain = differentail_output / differential_input

where:
differential_input = (IN+) - (IN-)
differential_output = (OUT+) - (OUT-)

Klaus
 

Upvote 0 Downvote
Thanks Everyone,it helped.
I have one more question related to differential amplifier design,

I am trying to Find SNR in ltspice but there is only noise analysis.
so i am thinking to calculate signal and noise power seperately. I have calculated the nosie (uV/sqrt Hz) over the frequency range (nearly60dB)
I am confused with the signal power. SHould i do transisent sim with sine wave and calculate the power?
 

Upvote 0 Downvote
Best way is probably to refer the noise back to the input and compare with the expected input signal.

Since you have a certain gain your output signal depends on the input signal amplitude and your SNR will only be valid for a certain input/output amplitude.

No need to run a transient. Run an AC to see your gain.

Note that your units for the noise is a bit strange. When you "calculate" noise over a frequency band your unit won't be V/sqrtHz. Also you mention that it is 60 dB, dB is unit less and that result doesn't make sense.
 

Upvote 0 Downvote
I am getting this noise plot, 60 dB was coming from 20log(Anoise)
. [IMG alt="
\mathrm{SNR_{dB}} = 10 \log_{10} \left [ \left ( \frac{A_\mathrm{signal}}{A_\mathrm{noise}} \right )^2 \right ] = 20 \log_{10} \left ( \frac{A_\mathrm{signal}}{A_\mathrm{noise}} \right ) = \left ( {A_\mathrm{signal,dB} - A_\mathrm{noise,dB}}\right ).
"]https://wikimedia.org/api/rest_v1/m...f2fbec646247d05d7d35efc260be9d2aa1bed3d[/IMG]
i am little confused now. Am i doing till the abeve step correctly?

Thanks
 

Upvote 0 Downvote
Okay then I understand where you got it from. The unit in this case is dBV, when you take 20log10(Volt) your unit is not dB but dBV.

But I would stop earlier in the equation, you have A_noise, just find A_signal and you're done.
 

Upvote 0 Downvote
martinnl said:
Okay then I understand where you got it from. The unit in this case is dBV, when you take 20log10(Volt) your unit is not dB but dBV.

But I would stop earlier in the equation, you have A_noise, just find A_signal and you're done.
Click to expand...
Thanks martinl,

In the schematic shown, i am planning to use cs stage with source degeneration to increase linearity.But as expected my gain is reducing. ANy suggestion on this?
 

Upvote 0 Downvote
Status
Not open for further replies.

Similar threads

Menu
Log in
Register
Cookies are required to use this site. You must accept them to continue using the site. Learn more…