samiran_dam
Full Member level 2
Dear All,
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)
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
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)
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