transimpedance amplifier

[nisha gupta]

hi
i m working on front end receiver. so LNA, transconductance amplifier, passive mixer ans transimpedance amplifier blocks are needed. except transimpedance anplifier other blocks are almost ready..i have designed TIA as well. but i need very low input impedance TIA (in the range of 30-50ohm)
but i getting around 170ohm. i have designed TIA using simple differential OPAMP (single stage) using RC feedback. can anyone suggest anything how to reduce input impedance or some topology in which i can get very low input impedance....
one more query .... i have integrated all blocks in front end receiver... i have calculated overall performance of receiver.. how can i measure individual block performance mainly linearity without adding buffer or balun at the output of individual blocks

 

[LvW]

i have designed TIA using simple differential OPAMP (single stage) using RC feedback.

can you show the corresponding stage (with feedback)?

 

[nisha gupta]

can you show the corresponding stage (with feedback)?

plz find the attaced circuit...

 

[LvW]

As far as I can see, your circuit approach is different from the classical approach for current-feedback amplifiers.
For example: Where is your high-impedance input?
Normally, CFA´s have a non-inverting high-impedance input node (base or gate) and an inverting low-impedance input (emitter or source) node for feedback purposes.

 

[nisha gupta]

As far as I can see, your circuit approach is different from the classical approach for current-feedback amplifiers.
For example: Where is your high-impedance input?
Normally, CFA´s have a non-inverting high-impedance input node (base or gate) and an inverting low-impedance input (emitter or source) node for feedback purposes.

hi
when i m calculating input impedance of the given circuit it is coming 167 ohm but i need around 50 ohms or less than this. so i used common gate topology. in that case also input impedance is not much low and output impedance is high enough. i want both input and output impedance both low.

 

[LvW]

hi
i want both input and output impedance both low.

In this case, you are not designing an amplifier that, normally, is called "transimpedance amplifier (TIA)".
An amplifier with all port impedances low-resistive is a current-controlled voltage source (CCVS, Norton amplifier).
As far as I can see, you are NOT using common gate topoogy. Both gates are used as signal inputs.

 

[FvM]

In my view, a TIA is an implementation of a CCVS by a feedback structure. A common gate stage can be understood as the most simple CCVS design (with respective limitations).

The OP seems to confuse input impedance of the non-feedback amplifier with TIA input impedance, which is provided by the feedback structure. The single stage differential amplifier shown in post #3 can be used as a differential TIA with the respective feedback network, but according the limited amplifier gain, the input impedance won't be particularly low.

Just mentioning 30-50 ohm versus 170 ohm input impedance is however very little of specification. Of course it's simple to achieve arbitary input impedances by adjusting feedback network and transistor area/current. But there will be surely specifications about transimpedance versus frequency, current consumption, noise. Without it, it's pointless to guess e.g. about the required number of amplifier stages.