The doubt in the circuit driven by P-MOS

Hello~all,thanks for reading my topic first !:slight_smile:

I’m a student majoring in electronics.I’m currently involved in a project about a circuit driven by P-MOS. My classmates and I am discussing about it and we have different opinions.So here we are.
Please look at the following picture:

In my opinion,this circuit just need a primary amplifier circuit.The transistor in series just want to increase the ability of driving.If the preceding amplifier circuit is deleted,the IO of SCM will connect R31 directly.If the outputted current of MCU is 1.2A,the behind transistor is breakover.I don’t think the preceding amplifier circuit is useful.
PS: Here the SCM is stm32,this is the datasheet of stm32. From the datasheet,it can be driven by high level and then outputted 1.2mA.
How do you think ? Please reply you opinion.

I can’t read the image, it is too small.

How about this ?

Why use a bipolar transistor? Since this is a switching application you can use another MOSFET (just make sure the gate breakdown voltage is >12V). You should do your engineering calculations based on the datasheets for the components you are using; if we tell you all about your circuit, what will you learn?

^^^ This!

However, I will help a little. What I would do is download LTSpice, and learn how to use it. Then enter in your schematic and simulate it; examining all the inputs and outputs of the semi-conductor devices.


Which explains the thinly disguised homework question.

That would be one hell of an MCU!

Don’t know where you get this figure either.

Although this question has nothing at all to do with KiCad, my opinion is that you need to do a bit more homework so that you could ask a more meaningful question then you might get more helpful replies. But I doubt anyone here want’s to do your homework for you.

1 Like

No, I’m NOT in the business of doing homework-for-hire but the Forum is being a little hard on this person. No it’s not a KiCAD question (perhaps Yahoo’s Electronics101 group, or even the LTSpice group are better choices) but he DID put the post in the “Community” section rather than a section that applies directly to KiCAD. The English is a little rough, and “1.2A” is probably a typo if you think about it, but engineers have worked around this kind of problem since the good Lord broke up the design group that was doing great things at Babel. (If the management at Babel had been better, we may not be in the current situation.)

Most important (to me), there is evidence that he HAS put some thought and effort into finding a solution, and he asked for confirmation that his solution is practical. That shows initiative and teachability.

Yes, that first transistor may be unnecessary. The two transistors are connected in a Darlington configuration. What are the advantages and disadvantages of Darlington pairs? What do your simulations tell you about this circuit’s operation - especially the current through R31? (What are the parameters of the input signal?) When you use known-good models of real-world transistors for the two NPN transistors, what are the effects of leakage currents?

Welcome to our Forum, @ling1995!


1 Like

While you weren’t forthcoming with any answers to his question in over 24 Hrs it took you only 2 to snap back at me. :wink:

Sorry, I was not aware there were sections of the “KiCad Info” forum that did not apply to KiCad.

Yes, it’s probably a typo. Between his text and his drawing I see 1.2A, 1.2mA and 12mA.

Perhaps you can read the Chinese on the drawing, you’re certainly reading something I don’t.

Perhaps. But in my experience people asking these sorts of questions are usually looking for answers, not lessons.

Or probably not. We could argue about it but we’ll never know without more information.

But I’m sure no one would object if you want to help with homework questions. :wink:

I think we can cut the guy a little slack, it’s not like we are inundated with homework questions.

It’s outside my area of expertise, so I was hoping to learn from the answers.

1 Like

Well no one else seems to want to offer their opinion regarding the above circuit and since @bobc has also indicated an interest I’ll go first.

If I understand the OP’s question he is asking if the circuit would still function if V2 where removed and the PWM signal applied to pin 1 of R31?

We don’t have enough information to fully analyse the circuit’s performance but in my opinion, not only could V2 be removed but the circuit would perform better without it.

Contrary to first appearances, V2 and V5 do not form a Darlington pair. Firstly they do not have common collectors and secondly R31 would defeat the purpose of any Darlington configuration.

V2 is simply an emitter follower and therefore the emitter voltage will always be 0.7V below the base voltage. Ignoring R27 and assuming the PWM input is a 3.3V signal we can see that the emitter voltage will never be greater than 2.6V (3.3V - 0.7V). The base voltage of V5 is 0.7V and this gives a voltage across R31 of 1.9V (2.6V - 0.7V) which results in 1.9ma (1.9V / 1000R) of base current.

It should be obvious at this point that with the PWM signal connected directly to R31 there would be 3.3V at pin 1 of R31 giving 2.6V (3.3V - 0.7V) across R31 or 2.6ma of base current. About 37% more than with V2.

Any further analysis is prevented by not knowing the gains of V2 and V5, or the PWM frequency, which would be necessary to determine if V3 was being switched fast enough and to calculate power loses of V3. But I assume that is beyond the scope of the OP’s question.

It would be nice of the OP could respond and let me know what mark I got. :grin: