MOSFET Push Pull Common Drain

[spikestar]

Hi everyone,

I have this following problem (uploaded it in the pdf). I have made an attempt at solving it, but now i am just stuck.

I have done question one, and have confirmed my answers in PSpice.

my Id = 16uA, Vgs = 1.4V and Vds = 4.6V ( 3 + 8/5)
and for p-channel the values are just opposite.

My problem is that i am stuck on question 2 where i have to calculate Vout min, Vout max, Vin min, Vin max. I thought that Vout max is Vdd - Vod, but i seem to be wrong.

Any help will be very much appreciated.

Kind Regards
spikestar

 

[uart]

How is the constant k_n defined Spikestar. There are two common definitions.

Is it I_d = 0.75 (Vgs-Vt)^2 or is it I_d = 0.75/2 (Vgs-Vt)^2 for the (approx) active region equation?Re the maximum voltage. Where it says "Calculate the max output voltage assuming both transistors stay in the active region", I would read that as find the input/output voltage at the point where either the pmos goes into cutoff or the nmos into triode, whichever is lower (for the positive voltage excursion). Start by finding where the pmos goes into cutoff as that's the easier of the two cases to solve.

 

[uart]

Thinking about this a little more I find the statement "assuming both transistors stay in the active region" to be a little troubling. Taking this literally then what I suggested above is correct. However the intention might have been simply that each driving transistor remain in the active region. In other words, that the n-ch device stays in the active region during +ive excursions and that the p-ch device stay in the active region during -ive excursions.

Edit: No I think the first (literal) interpretation was correct as the circuit is approx linear if both mosfets stay in the active region at all times. I'm more used to BJT "push pull" stages where the non-driving device definitely goes into cut-off, but after considering the mosfet push-pull stage in a bit more detail I see that it does work much better if both devices stay active over the full cycle.

Skipestar. Can you workout the maximum Vi-Vo differential (for +ive Vi) before the p-ch device goes into cut off?

 

[josepak]

Hi spikestar,

I'am doing the same assignment as your question, but i don't know how to do question 1, can anyone show me how to do question 1?

Thank for help.

 

[rezeew]

Hello spikestar,

The MOSFET push-pull common drain circuit is a type of amplifier circuit commonly used in audio applications. It consists of two MOSFET transistors, one p-channel and one n-channel, connected in a push-pull configuration. This allows for high output power and low distortion.

In regards to your problem, it seems like you have made a good attempt at solving it. However, there are a few things to keep in mind when calculating the values for Vout min, Vout max, Vin min, and Vin max.

First, Vout min and Vout max refer to the minimum and maximum output voltages, respectively. These values can be calculated using the equation Vout = Vdd - (Vod + Vds), where Vdd is the supply voltage, Vod is the overdrive voltage (the difference between Vgs and the threshold voltage), and Vds is the voltage drop across the drain-source junction of the MOSFET.

Second, Vin min and Vin max refer to the minimum and maximum input voltages, respectively. These values can be calculated using the equation Vin = Vgs + Vth, where Vgs is the gate-source voltage and Vth is the threshold voltage.

It is important to note that the values of Vout min, Vout max, Vin min, and Vin max will vary depending on the specific characteristics of the MOSFET transistors used in the circuit. Therefore, it is important to use the correct values for Vod and Vth in your calculations.

I hope this helps and good luck with your problem-solving!