Drain voltage and current

Drive capability of a CS stage

The most uncomplicated method of creating an amplifier is to use a single CS stage. However, in order to test if this one stage is able to satisfy the requirements, some drive capability and noise simulations have to be done. The stage has to be able to drive the load without adding too much noise.

First, the correct biasing needs to be determined to fulfil the design requirements. The active antenna needs to deliver $0dbm$ ($1mW$) into $50 \Omega$. The RMS voltage should be $V_{RMS}=\sqrt{P \cdot R} = 233 mV$. Now the peak value is obtained by multiplication with the crest factor $V_{PP}=\sqrt{2}\cdot 233mV = 316 mV$.

Thus the CS stage needs to drive $316 mV$ at the output. The first method is using brute force by placing an additional $50\Omega$ resistor in series witth the amplifier output. Then the total impedace seen at the output would be $100\Omega$. In order to fullfil the $316mV$ requirement we need a current of $$I_D= 316mV/50\Omega = 6.32mA$$ This means about $6.4mA$ is necessary to drive the output stage.

On the other hand, the CS stage needs to drive $-316 mV$ at the output as well. Using the same brute force method, the drain-source voltage of the stage should drive $2\cdot 316 mV$. This means that the voltage necessary to drive the load is: $$V_D = 632 mV$$

Using the testbench provided for the driving capability, the following output characteristics are obtained for the drive voltage.