# Amplifier 2nd second stage topology clarification

#### ChetanGK

##### Newbie level 6
Hi all,

I am designing a front end for capacitive sensing application and mixer for lock-in demodulation. The mixer is double balanced voltage mode passive mixer (50% duty cycle) and LPF filter is differential RC filter for cut off frequency of 1KHz. Please find the attached image(system.png)

One of the mixer input is connected to amplifier output and the other input of the mixer is connected to reference voltage. The mixer's differential output is connected to differential RC LPF. The equivalent circuit (not sure though) can be seen from image (equivalent_load.png).

I am at stage of decide the second stage topology for my front end amplifier. As it is not typical capacitive load, I am not sure about what topology fits my application.

Amplifier specification(preliminary)
GBW ~ 400Mhz (based on Noise gain)
F3db = 20MHz (closed loop)
Noise gain ~ 15 (Cf = 60f , differential sensor cap = 700f )
Acl = 0.5
Aol = 65dB

Can someone please guide how to analyse the load for the amplifier and topology for second stage?

Regards,
Chetan

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#### t4_v

##### Full Member level 2
Where exactly would you like to put the 2nd amplifier?

#### shanmei

The Acl is GBW / F3db =400MHz/20MHz=20, why Acl = 0.5 ?

#### ChetanGK

##### Newbie level 6
@t4_v _c
I wanted to decide on the second stage of 1st amplifier ...(meaning Class AB output stage or Class A stage)

@shanmei
The sensor at the input of amplifer is differential sensor. The difference in charge is being sensed. The total cap seen by the amplifier is summation of 2 sensors (ref capacitor and sense capacitor),hence the f3db of close loop system: F3db = F_gbw * Cf/(Cf + Cref + Csense)
If I need f3db to be 20Mhz , then with my sensor values I need GBW to be 400Mhz.

Reference: Analysis of non-ideal characteristics of an ac-based capacitance transducer for tomography