In the previous post on MOSFETs , I introduced how PMOS and NMOS Structure and Characteristic functionality works. This post is based on the MOSFET prerequisites, which introduces some of the important ASIC design concepts as listed below.



Here, I introduce the concept of Small Signal. Firstly, before we do this analysis, we need to bias the transistor.

Biasing means we need to set the work-point/operation-point of the transistor. So, 'Vs' in FIG 1 is our small signal, which means that the amplitude is low, say 1mV. Since we have a DC (FIG 1) along with AC as well, so the Gate Voltage has a DC and AC value at the same time.





Let us, for a while, forget about this Small Signal and let us bias the transistor by applying a bias or positive voltage to the Gate. In the above circuit, the load R L is connected to the source voltage VDD.

Now, if you look at FIG 2, we have chosen an operating point (Q point). To choose this point, we choose V GS . Also, by changing the transistor size (W/L), we are gating the current (I DS ) from the transistor.





Let us say, the long channel is high, and the lambda value is 0. Assuming (from the lower V GS to higher V GS value) that the constant current curves have value:- V GS1 , V GS2 , V GS3 . Thus, we are setting V GS2 as the V GS . Now we can set-up the DC point for the current value (I D2 ). With all this, we can get the V DS by using this: V DS = V DD - R L .I D . Thus, the V DS value is on the X-axis (marking on the X-axis), and we can assume it as V D2 . The intersection of I D2 and V D2 is the bias point, and here, the transistor is working in the saturation region. In this region, V DS > V GS - V T and, V GS2 > V T which turns-on the transistor.





Now, let us consider the AC signal and assuming the time-varying voltage component with its amplitude as 1mV. Thus total V G = V GS2 + V S = 1V + 1mV cos(wt). Refer FIG 1. We have the alternating signal for the Gate, Drain, and Current I D on R L .





To show this alternating small signal on the graph (FIG 2), we will now consider the top and bottom limit of AC voltage signal which would be [1V + 1mV] and [1V - 1mV] respectively. Similarly, we are going to have fluctuation in the current values below and above I D2 which can be named as I D1 and I D3 . We can also mark the above and below of the chosen DC voltage value V D2 for the AC voltage as V D1 and V D3 . The current and voltage values are equal to I D3 - I D2 . and V DS3 - V DS2 .





So as per the small-signal analysis, we can see if we apply the AC Voltage and DC Voltage to the Gate of the transistor, we will be able to get the varying current and drain voltage and they are all small signal.





Small Signal Model (Analysis):









g m = ( δ I ds ) / (δ V gs ) : Fluctuation of current over voltage of gate-source.

= ( g m = √ [ 2 . μ n .C ox (W/L) . I DS ]

= .C (W/L) . I ] g m = 2I D / (V GS - V T )

g m = [( I ds3 - I ds1 ) / ( V gs3 - V gs3 )]



