Biasing Scheme for CMOS Telescopic Amplifier


Telescopic amplifier has higher DC gain due increased output resistance because of cascode transistor. The gain improvement is obtained at the cost of lower output swing. The output swing limitation comes from more number of devices in the stack and also due to biasing scheme employed for cascode transistors. Some of the popular biasing schemes are discussed here.

Scheme-1

Telescopic cascode operational amplifier biasing scheme

Consider a telescopic cascode amplifier with the biasing scheme as shown in Figure 1. The right and left half circuits are identical, therefore they carry equal currents I_1 = I_2 = I_{ref} at the DC operating point. In this analysis secondary effects, like channel length modulation or body effect, are not considered.

Let V_{ovi} is the overdrive required for i^{th}-transistor (M_i) to carry a current equal to I_{ref} in saturation region. Then

(1)   \begin{equation*} V_{x} = V_{dd} - ( 2 V_T + 2 V_{ov5} + V_{o3} ) \end{equation*}

Nominally V_{out} is same as V_{x} due to circuit similarity and identical biasing conditions. Even the intrinsic feedback of the circuit forces V_{out} to be nominally same as V_{x}

V_{out} = V_{G6} = V_{G8}.
The output swing in positive direction is limited by M6 and negative direction by M8 by going into linear region.
The upper limit of V_{out}
V_{out,max} = V{G6} + V_T = V_x + V_T

V_{out,min} = V{G8} - V_T = V_x - V_T

The limits on the output swing are

    \[ V_{x}-V_T < V_{out} < V_{x} + V_T \]

Therefore, the total output swing is limited to \pm V_T around DC operating point. The upper swing can be improved by cascode current scheme and lower swing by some other technique.

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