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Need help on Rail-to-rail input

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dra66ard

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Hi, I am currently design an rail-to-rail op amp and i found a circuit for the input stage with rail-to-rail input using complementary differential amplifier but unfortunately there are additional transistor in the circuit (those transistors that i circle in red). I know the additional transistors are for maintaining the gm in the circuit but i do not know how is the circuit works after the differential amplifier. Can anyone please help to explain to me ? Your kindness is highly appreciated and thank you in advance =)
 

The transistors in red boundary are nothing but a simple single stage opamp. The final transistors(common source ones to the right) are used to add the current from the pMOS and nMOS differential inputs.
 
ohh..thank you for ur reply and information. Because from what i found from the journal, it said that this is complementary differential input with active load control. The additional 6 transistor for the upper and lower (circle in red) is to constant the transconductance / gm value. Are they current mirror? coz it looks like current mirror configuration. Thank you in advance for any help. =)
 

Hi,

If MOS names are wrong, sorry, it's hard to read... I think NP2 + NP4, and NN3 + NN4(?) form two current mirrors. MP5 and NN5 (or does that say 6?) look like a summing output stage. To me, it looks like a two-stage op amp, rail-to-rail complementary N and P input pairs, and a complementary r-to-r second stage, from the little I know the second stage does the job of gain stage and output stage, or possibly the gain stage is not necessary and the circuit goes from diff. pairs to output stage directly. I could be quite wrong...
 

possibly the gain stage is not necessary and the circuit goes from diff. pairs to output stage directly.
Hardly possible if you want some output voltage range, or even rail-to-rail output.

The circuit isn't more than the very basic idea of an OP with rail-to-rail input, but it doesn't work well as is. If the input voltage approaches positive or negative rail, one current mirror shuts down and the output stage looses its bias current.
 
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    d123

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oh, thank you for the information. here is my full circuit, where on the left is consisted of current bias circuit which supply current to the input and output, and also class AB amplifier for the output stage.
fullcircuit.jpg

- - - Updated - - -

hi, sorry to disturb again.Please ignore my post above. This is my new circuit below, those two transistors circled in red below, the two transistor does make the transconductance (gm) constant by acting as current switch? thank you =) sorry for the hassle, i m not good in circuitry
fullcircuit_eda.jpg
 

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