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Discuss Could somebody please tell me what the name of the below circuit is please in the UK Electrical Forum area at ElectriciansForums.net
![[ElectriciansForums.net] Could somebody please tell me what the name of the below circuit is please](https://electricians.s3.eu-west-1.amazonaws.com/data/attachments/72/72008-ea76cc5c5c29cf5bd5c837fa6f5307c8.jpg)
Where did you get the image from? I suspect a website. In which case that would be the best source of explanation for it.
Last edited:
Either that, or it's someone's homework !
TBH, I sort of missed out learning about MOSFETs, but I recognise this as the classic single transistor inverting class A amplifier circuit - but with a MOSFET instead of an NPN bipolar transistor.
C is just DC blocking.
In the absence of an input signal :
R1 & R2 bias the gate to a set voltage, we'll call it Vg.
To a rough approximation, the transistor will then turn on until the current through Rs causes the gate-source voltage to stabilise at a value that will cause the transistor to conduct that much current. With a silicon bipolar transistor, you'd use around 0.6V as a starting point for the base-emitter voltage, dunno what the equivalent for a MOSFET is.
The current through Rs also comes through Rd, causing Vout to take up some DC voltage in between Vdd and Vss.
If we now apply a rising voltage to the input, that will increase the voltage on the gate, the transistor will turn on a bit more, the current down the Rd-transistor-Rs path will increase until (approximately) the voltage across Rs increases to match the change in Vg. The increase in current also causes an increase in volt drop across Vd, so Vout reduces. The change in Vout for a given change in Vin is roughly proportional to the ratio of Vd to Vs - so if Vd is (say) 5 times the resistance of Vs, then the gain will be approximately 5, or more correctly, -5 as it's an inverting circuit.
These circuits are generally only used for small signals, otherwise they waste a lot of power. The transistor is always conducting (unless you feed in a very large signal, in which cause you'll get an 'orrible sound out if it's an audio amplifier), and there's always current flowing down the Rd-transistor-Rs path which generates a lot of heat - much more than the usable output power.
That's all dredging up stuff I learned "a few" decades ago and haven't used for "several" decades. So feel free to point out if it's all wrong.
TBH, I sort of missed out learning about MOSFETs, but I recognise this as the classic single transistor inverting class A amplifier circuit - but with a MOSFET instead of an NPN bipolar transistor.
C is just DC blocking.
In the absence of an input signal :
R1 & R2 bias the gate to a set voltage, we'll call it Vg.
To a rough approximation, the transistor will then turn on until the current through Rs causes the gate-source voltage to stabilise at a value that will cause the transistor to conduct that much current. With a silicon bipolar transistor, you'd use around 0.6V as a starting point for the base-emitter voltage, dunno what the equivalent for a MOSFET is.
The current through Rs also comes through Rd, causing Vout to take up some DC voltage in between Vdd and Vss.
If we now apply a rising voltage to the input, that will increase the voltage on the gate, the transistor will turn on a bit more, the current down the Rd-transistor-Rs path will increase until (approximately) the voltage across Rs increases to match the change in Vg. The increase in current also causes an increase in volt drop across Vd, so Vout reduces. The change in Vout for a given change in Vin is roughly proportional to the ratio of Vd to Vs - so if Vd is (say) 5 times the resistance of Vs, then the gain will be approximately 5, or more correctly, -5 as it's an inverting circuit.
These circuits are generally only used for small signals, otherwise they waste a lot of power. The transistor is always conducting (unless you feed in a very large signal, in which cause you'll get an 'orrible sound out if it's an audio amplifier), and there's always current flowing down the Rd-transistor-Rs path which generates a lot of heat - much more than the usable output power.
That's all dredging up stuff I learned "a few" decades ago and haven't used for "several" decades. So feel free to point out if it's all wrong.
That's what I'm beginning to think with these questions - it may be a homework thing. Otherwise why just post circuits and ask what they are for.
Apologies to the OP if that's not the case.
It's a common-source amplifier, which can be identified by the input being applied to the gate and the output being taken from the drain.
There's a lot more I could say but as already mentioned, it would be nice to know some context to these questions. We like to help, but now you know what to look for, you'll find this standard circuit element with analysis in the first few pages of any relevant text-book chapter describing the operation of FETs, so there's no point in us re-writing it from scratch on the forum.
If we can help analyse specific examples and problems that you might be having then please tell us about them and we can be of more direct and specific assistance.
There's a lot more I could say but as already mentioned, it would be nice to know some context to these questions. We like to help, but now you know what to look for, you'll find this standard circuit element with analysis in the first few pages of any relevant text-book chapter describing the operation of FETs, so there's no point in us re-writing it from scratch on the forum.
If we can help analyse specific examples and problems that you might be having then please tell us about them and we can be of more direct and specific assistance.
OP
Hi all, thank you very much for the responses. I did post the context of the questions yesterday but it doesn't seem to have posted. It is part of my current course work, the problem I have is because its an online based course the notes I have basically explain each component. When it then comes to trying to work out what the circuit does I find it difficult so was just reaching out for an idea really. Apologies if this is not allowed or frowned upon on here, but you guys really do help me get my head around it as I then have an idea of what to search for etc to further understand these circuits.
OP
I wish lol
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