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Chapter 5

Q. 5.6

65nm CMOS chain scaling

Consider a 6-stage CMOS amplifier chain where each stage has minimum gate length transistors, with W=20 \mu \mathrm{m} and 20 unit fingers, each 1 \mu \mathrm{m} wide and contacted on both sides of the gate. R_{D}=200 \Omega. Each stage is biased at 0.15 \mathrm{~mA} / \mu \mathrm{m} and the corresponding small signal parameters of the MOSFET are R_{g}=10 \Omega,\left(200 \Omega / N_{f}\right) C_{g s}^{\prime}=0.7 \mathrm{fF} / \mu \mathrm{m}, C_{s b}^{\prime}=C_{d b}^{\prime}=0.7 \mathrm{fF} / \mu \mathrm{m}, C_{g d}^{\prime}=0.4 \mathrm{fF} / \mu \mathrm{m}, g^{\prime}{ }_{m e f f}=0.9 \mathrm{mS} / \mu \mathrm{m}, g_{\text {oeff }}^{\prime}=0.18 \mathrm{mS} / \mu \mathrm{m}. (a) Calculate the DC gain, the 3 \mathrm{~dB} bandwidth and the gain-bandwidth product of the individual stage and of the amplifier chain. (b) Re-design the amplifier chain using size and bias scaling by a factor of 2 from the input to output, knowing that the first stage has R_{D}=62.5 \Omegaand W=64 \mu \mathrm{m}. Assume the same bias current density, and small signal parameters per unit gate width as in the original amplifier.


Verified Solution

The low-frequency voltage gain per stage is

A_{S}=-g_{\text {meff }}\left(r_{o}|| R_{D}\right)=-0.9 e-3 \times 20 \times 116=-2.088 \\


\tau_{p}=\left(R_{D} \| r_{o}\right)\left[C_{d b}+C_{g s}+\left(2+\left|A_{0}\right|\right) C_{g d}\right]+R_{g}\left[C_{g s}+\left(1+\left|A_{0}\right|\right) C_{g d}\right]=7.55 p s \\


B W_{S}=\frac{1}{2 \pi \tau_{p}}=21.1 G H z


G B W_{S}=44.05 \mathrm{GHz}

For the chain, A_{t o t}=2.088^{6}=82.86=38.36 \mathrm{~dB}, B W_{t o t}=\left(\sqrt{2^{1 / 6}-1}\right) 21.1 e 9=7.38 \mathrm{GHz} and G B W_{\text {tot }}=632.9 \mathrm{GHz}.

An exact computer simulation using the design kit models gives A_{S}=6.4 \mathrm{~dB}(2.089), B W_{s}=18.96 \mathrm{GHz}, and A_{\text {tot }}=38.37 \mathrm{~dB}, B W_{\text {tot }}=6.2 \mathrm{GHz}. The error between the hand calculations and exact simulation is 10 % for the stage and about 16 % for the entire chain.
(b) If we use scaling by  k=2 : W_{1}=64 \mu \mathrm{m} , R_{D 1}=62.5 \Omega , W_{2}=32 \mu \mathrm{m}, R_{D 2}=125 \Omega, . . . , W_{6}=2 \mu \mathrm{m} , R_{D 6}=2 \mathrm{k} \Omega

The gain per stage remains unchanged, but the time constant and bandwidth of the stage change to

\tau_{p} =\left(R_{D} \| r_{\mathrm{o}}\right)\left[C_{g d}+C_{d b}+\frac{C_{g s}}{k}+\left(1+\left|A_{0}\right|\right) \frac{C_{g d}}{k}\right]+R_{g}\left[C_{g s}+\left(1+\left|A_{0}\right|\right) C_{g d}\right]\


=36.25[25.6 \mathrm{f}+44.8 \mathrm{f}+22.4 \mathrm{f}+3.1 \times 12.8 \mathrm{f}]+3.125[44.8 \mathrm{f}+3.1 \times 25.6 \mathrm{f}] \\


=4.9 \mathrm{ps}+0.388 \mathrm{ps}=5.288 \mathrm{ps}

We obtain B W_{S}^{\prime}=30.1 \mathrm{GHz}, almost a 50 % increase in stage bandwidth compared to the original amplifier, and B W_{t o t}^{\prime}=\left(\sqrt{2^{1 / 6}-1}\right) 30.1 e 9=10.53 \mathrm{GHz}.

Using computer simulation, we obtain B W^{\prime}_{S}=28.17 \mathrm{GHz},B W_{\text {tot }}^{\prime}=8.73 \mathrm{GHz},G B W^{\prime}{ }_{S}=59.15 \mathrm{GHz},G B W_{\text {tot }}^{\prime}=748.7 \mathrm{GHz}

Again, the error between hand calculations and computer simulations using design kit models is within 10 % for the single stage but increases to 17.1 % for the entire chain.

Note that, if we consider interconnect capacitance and the parasitic capacitance of the load resistor, the calculated bandwidth will be smaller because the interconnect capacitance is comparable to that of the smallest size stage.