#### samiran_dam

##### Full Member level 2

I am designing a OTA-C Biquadratic Filter (Butterworth Approximation) stage. The topology is:

For this topology, filter cut-off frequency and Q-factor is defined by:

**2*π*ω=gm/√(c1*c2) [gm1=gm2=gm]**

Q=√(c2/c1)

Q=√(c2/c1)

For

**200 KHz**filter cutoff frequency and

**Q=1.93**, I calculated

**c1**and

**c2**values as

**15.67 pF**and

**58.36 pF**respectively given

**gm=38.17 uA/V**. When I simulated the circuit with a simplistic OTA model (with non-zero output conductance and non-zero output parasitic capacitance included in the model), it produced desired values of cut-off frequency and Q factor.

But when I realized a transistor-level circuit, although filter cut-off frequency is

**199.98 KHz**, Q is

**1.85**. From the DC analysis I have confirmed that the gm value is exactly 38 uA/V for the transistor-level OTA used. And the output conductance considered in the model-based simulation are same as the output-conductance delivered by the transistor-level circuits. Q is simply defined the ratio of two external capacitors. I am unable to identify which non-ideal factor of the OTA is affecting the Q to be degraded. If somebody has any idea about this, please help me.

regards

**Sam**