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# npn transistor in circuit

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#### zarric

##### Junior Member level 3
npn transistor circuit

who can tell me how to understand the npn used?

npn transistor circuits

As the schematic is given the given transistor should be PNP. NPN transistors will not conduct to activate the load if the base is grounded. Good luck

npn circuit

zarric said:
who can tell me how to understand the npn used?
It's a base stage, like the upper part of a cascode stage (drawn upside down). Could serve as an impedance converter (low input impedance, high output impedance (Rout=Rload)), but for relatively small signal only: The emitter being at -0.7V (in case of Si), the collector could have something in between, say -0.45V, hence the max. output swing could be something like ± 200 .. 250mV . It has some voltage gain, current gain ≤ 1, no phase inversion.

npn transistor impedance

The low input impedance would suggest an Iin rather than a Vin label at the node apparently intended as input. Also Vin must be negative related to gnd(and would be typically drawn near the schematic bottom).

It's hard to decide, if the circuit is dedicated to the purpose analyzed by erikl or just a drawing artefact... Using pnp instead of npn wouldn't change anything, except voltage and current polarity.

impedance npn transistor

erikl said:
It has some voltage gain ...
The voltage gain vu , in good approximation, is vu = 25/V * V(Rload) ,
with the above assumption: vu = 25/V * 0.45V = 11.25 ≈ 10

FvM said:
The low input impedance would suggest an Iin rather than a Vin ...
With the same approximation, and Ic = V(Rload)/Rload
Rin = ΔVbe/ΔIe ≈ 1/25 [V/Ie] ≈ 1/25 [V/Ic] = Rload/(25*0.45) ≈ Rload/10

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