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Below is the circuit in which a PNP Current mirror circuit is used to control a P- channel Mosfet. What is the intended operation of this circuit? V2 acts a power supply voltage for many devices down the line.

  1. Is the MOSFET intended to be in ohmic region or saturation?

  2. What will be the voltage at V2?

  3. Does this still work as current mirror?

schematic

simulate this circuit – Schematic created using CircuitLab


Edited section:

I have read about ideal diode and understood the following:

If V2 < V1 (10V), it implies

i) Vbe(q2) < Vbe(q1) => Ic(q2) < Ic(q1) => Vc(q2) < Vc(q1)

ii) Q1 just acts like a diode clamped to 0.7V, Q2 acts like a transistor in active region

iii) Some sort of regulation happens with Iq2 and V2. As V2 dips ( say due to a load increase), Q2 conducts less=> M1 is driven harder=> V2 increases.

Iam unable to find to what voltage it gets regulated to? If it has be an ideal diode, somehow it should get regulated very close to V1.

iv) As V2 dips further , Q2 enters enters in cutoff and MOSFET M1 is fully turned ON. After that V2 =V1- I*Rdson

Iam not sure whether the body diode would ever conduct. Also Iam not seeing any current mirroring happening here. May be I correlated this to current mirror wrongly?

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  • \$\begingroup\$ What do you think happens when V2 >10V, <10V and open circuit? \$\endgroup\$ May 8, 2020 at 4:58
  • \$\begingroup\$ Added the reply in edited section \$\endgroup\$ May 8, 2020 at 9:08

2 Answers 2

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The circuit approximates an ideal diode, it is described in detail here: Ideal Diode If Vin>Vo, M1 is driven hard on. If Vin

I mostly agree.

Q1 in this configuration is called a diode connected BJT, its behaviour is slightly different to a diode but your reasoning seems sound.

Regarding the regulation of V2, to determine the relationship between V2 and iLoad (load regulation) you might consider that the Q1,Q2,R1,R2 circuit is a differential V amplifier, the output V of which drives M1 in its linear mode (approximating a Voltage controlled R). So the load regulation will depend on the amplifiers V gain together with M1's Vgs,Rdson specs.

No, I dont believe this is a current mirror...closer to a 'long tailed pair'?

I agree, the body diode is redundant.

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  • \$\begingroup\$ Thanks for the link \$\endgroup\$ May 8, 2020 at 6:29
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So we know can make better, faster, Full Wave Rectifiers using a couple opamps and a couple of these circuits.

Let this circuit struggle with the speed requirements, instead of burdening the opamps and the slewrate limitations.

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