JNTU
JNTU B.Tech IV ANALOG CMOS IC DESIGN Previous Question Paper (July - 2021 - R16)
Answer any Five Questions. All Questions Carry Equal Marks.
Main
Deduce the small signal model for an n-channel MOSFET taking into account the body effect.
Answer
The small-signal model of an n-channel MOSFET including the body effect incorporates the transconductance of the body-source junction. The body effect occurs when the source is not connected to the substrate, causing the threshold voltage to vary with the source-to-body voltage (Vsb). This is modeled by adding a voltage-controlled current source in parallel with the main drain-source current source.
The current source representing the body effect is defined as:
where g_mb is the body transconductance, given by:
The complete small-signal model includes the gate-source voltage (Vgs) controlling the main current source (g_m * V_gs), the body-source voltage (Vbs) controlling the secondary current source (g_mb * V_bs), the output resistance (r_o), and the parasitic capacitances (C_gs, C_gd, C_sb, C_db).
Derive an expression for gm of an N-channel MOS FET operating in linear and saturation regions.
Answer
The transconductance (g_m) is defined as the partial derivative of the drain current (I_d) with respect to the gate-source voltage (V_gs), keeping the drain-source voltage (V_ds) constant.
1. Saturation Region: In saturation, the drain current is given by:
Taking the derivative with respect to V_gs:
Alternatively, expressed in terms of drain current:
2. Linear (Triode) Region: In the linear region, the drain current is given by:
Taking the derivative with respect to V_gs:
This shows that in the linear region, transconductance is directly proportional to the drain-source voltage (V_ds) and the device geometry, independent of the gate-source voltage.
Explain how to realize Resistor using Active devices.
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Consider a source follower which is biased by a current mirror. The dimensions of all the transistors are 100µm/1.6µm, µnCox = 90µA/V2, µpCox = 30µA/V2, Ibias = 100µA, γn = 0.5V1/2, rdx-n = 8000L(µm)/ID (mA). What is the gain of the stage?
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Explain how power-supply sensitivity can be reduced in bootstrap bias technique.
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Design a second order Butterworth low pass filter with a cutoff frequency of 500 Hz and a pass band gain of -2. Assume that a 5V ± power supply and a CMOS clock are used.
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Why is emitter resistor RE replaced by a constant current bias circuit in differential amplifier stage of an op-AMP? Explain.
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Design the dual input balanced output differential amplifier with the current mirror bias according to the following specifications: i) Supply voltage Vs = ±12V ii) Maximum output voltage swing – 7Vpp.
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Discuss in detail the compensation of OP amp that makes it completely independent of process and temperature variations.
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With neat sketch and necessary equations explain the design aspect of a two-stage open loop comparator for slewing response.
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Give an account of charge injection errors in connection with comparators and suggest a method to minimize the same.
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Explain about the Bipolar simple current mirror with degeneration helper with necessary equations.
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Determine the DC gain and 3-dB bandwidth frequency for the circuit with the load impedance of 150kΩ || 20pF added to the output.
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Choose values of VGS = 1, 2, 3, 4 and 5V, assume that the channel modulation parameter is zero. Sketch to scale the output characteristics of an enhancement n-channel device if VT = 0.7V and ID = 500μA when VGS = 5 V in saturation.
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