[SOLVED] Active filter design

[electronhole]

I want to design a active filter emulating the RLC filter.
I have two options to implement
a) using gm-C
b) using op-amp and RC

which is a better method and why??
also i wanted to know how the frequency responses are affected due to parameter variations in both the cases.. i want to know the underlying theory..
Thanks!

 

[LvW]

In general, it is not possible to ask "which one is better ?" without knowing more details about
* filter type (low/high/band pass)
* required filter degree
* frequency range (pole frequencies)
* Selectivity requirements (pole Q values)
* required input/output impedances (limits)
* tunability requirements
* allowed number of IC`s (limits)
*
*
The background is the fact that every design in electronics consists of a trade-off between conflicting requirements.

 

[jiripolivka]

Find a textbook on active filters with opamps. There are many, and here it is no room enough to answer all your questions.

 

[electronhole]

well, when emulating the inductor (of parallel R L C ) by active elements we do have both bandpass and lowpass nature at both nodes in a biquad section.

I'm implementing a buttreworth response with wp = few MHz

In particular, is it possible to tell which one (gm-C or op-amp RC) will be more immune to capacitance variation at each and every node present in the circuit??

 

[crutschow]

What type of transconductance amp were you thinking of using at a few MHz?

 

[LvW]

Active L-simulation in gm-C technique can, in general, not to be recommended.

 

[electronhole]

can you please explain the reason?

 

[LvW]

can you please explain the reason?

An active simulation of an ideal inductor is not possible. That means: We have to accept some errors caused by the imperfections of the active device.
These errors are much smaller for opamps if compared with OTA`s.

 

[electronhole]

and whats the reason for the errors to be small in case of op-amps compared to OTA

 

[LvW]

In most cases, it is the finite output resistance of the OTA (ideal: infinite) which introduces errors. More than that, for voltage-mode operation additional buffers are required.
Primarily, OTA`s are used for inegrated analog signal processing (less space than opamps).