Diary of a Trainee Electronics Engineer: February 2018Follow article
Transistor circuit calculations and visiting Bradford College.
In last month’s post, I mentioned that there has been quite a lot of overlap between the two units, Analogue System Integration (ASI) and Digital System Integration (DSI). As we have got further into the second term we have gone further into the detail of transistor circuit calculations.
One thing I’ve really enjoyed is the tutorials which run with DSI following the lecture; here we have a chance to learn the equations and practice the calculations for the various types of transistor circuits discussed.
In particular, we looked at CMOS transistors, which are made of complementary and symmetrical pairs of NMOS and PMOS transistors. Much of the time, we are required to use the information given in a question to determine the width to length ratio, of the P-type and N-type MOSFET transistor in the circuit given a certain propagation delay.
CMOS Transistor Calculations
Here is an example of one of the questions and solutions which came up in one of the last DSI tutorials.
Design an asymmetrical CMOS inverter with (W/L)P = (W/L)N to provide a propagation delay of 400 ps for a load capacitance of 200 fF. Use VDD = 2.5 V, K’n = 100 μA/V2 , K’p = 40 μA/V2 ,VTN = - VTP = 0.60 V.
To begin with solving this we use the following formulas:
Below are the formulas to determine RonP and RonN respectively:
Knowing this we can substitute the values stipulated into the question in the equations. First we will start with:
Next we do the same for the equation:
Having calculated TPLH and TPHL, we can now substitute these values into the equation for TP:
From the question we know Tp=400x10-12
We can simplify this by multiplying the left-hand side by the denominator of the right-hand side of the equation.
As this is an asymmetrical inverter we know that (W/L)n will be equal to (W/L)p , so we can change the denominator of (W/L)p to (W/L)n so that they are equal.
With the denominators the same we can further simplify this to find (W/L)n .
As I mentioned previously, because this is an asymmetrical inverter (W/L)n will be equal to (W/L)p meaning that:
Once you get the hang of these calculations they aren’t that bad. It takes a little bit of a practice and remembering when and where to use the appropriate formulas, as they vary for different types of transistor circuits.
Student Profile Video
Just before Christmas I was approached by a member of the Higher Education Marketing Communications team and asked if I’d be willing to participate in a video about my time as a student at Bradford college, which would be used for promotional purposes. So around the beginning of the month, I took a trip over to Bradford to take part. Overall it was a great day and it was really nice to visit Bradford College again since leaving to top-up to my Degree!
As usual, it’s been another productive month as a Trainee Electronics Engineer. As we get closer to spring the exam season is fast approaching, so I can see the next month being crammed full with revision sessions and beginning to finalise course content. Let’s see what next month holds!