Long tailed pair collector current mirror

Q

Quasar999

New Member

• Apr 7, 2021

• #1

Hello
i want to understand how work a differential amp with a mirror current section and i did the following pspice simulation. Is really weird cause the mirror current at the top keep the same current in the Q1 e Q2 have both the same base current and is not clear how change the Vce in them.
From the simulation output voltage seems to have a good gain but is not well biased. What do you think?

 

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• DiffAmp.jpg

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crutschow

Well-Known Member

Most Helpful Member

• Apr 7, 2021

• #2

You .asc file (LTspice, not Pspice.) is different that shown in the .jpg file.

 

unclejed613

Well-Known Member

Most Helpful Member

• Apr 7, 2021

• #3

the output of a diff amp is in the form of current, not voltage. to get voltage gain out of it, you need a voltage amplifier stage to transform the current variations back into voltage variations. this would be a PNP transistor with the emitter tied to the +vcc rail, the base to the diff amp's out-, and a 20k or so resistor from the collector to the -vcc rail... you also do not need 0.8v dc bias on the diff amp inputs, as the emitter current source will float the emitters slightly negative...

 

Q

Quasar999

New Member

• Apr 7, 2021

• #4

Updated Schematic. Added the pnp at the output but still the result are weird.

 

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• Draft4.asc

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crutschow

Well-Known Member

Most Helpful Member

• Apr 7, 2021

• #5

Is this what you expected?
The circuit has a very high gain (over 80,000 to Out2), so is sensitive to very small offsets in the circuit (apparently about 1mV referred to the input here).

 

Last edited: Apr 7, 2021

crutschow

Well-Known Member

Most Helpful Member

• Apr 7, 2021

• #6

unclejed613 said:

this would be a PNP transistor with the emitter tied to the +vcc rail, the base to the diff amp's out-, and a 20k or so resistor from the collector to the -vcc rail

Click to expand...

The PNP's form a current-mirror differential to single-ended converter and require no resistors.

 

Q

Quasar999

New Member

• Apr 8, 2021

• #7

Thx. How i could try to remove the offset?

 

Q

Quasar999

New Member

• Apr 8, 2021

• #8

What do you think?

 

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Last edited: Apr 8, 2021

crutschow

Well-Known Member

Most Helpful Member

• Apr 8, 2021

• #9

Quasar999 said:

What do you think?

Click to expand...

Looks good.
But note that the simulation uses identical (matched) transistors, while the real circuit with discrete parts will have offsets due to the normal mismatch been transistors of the same part number.