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How about this bandgap current source?

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Davidy

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Who can help me the analysis this circuit?
VDD＝3V and 'I50A' should be 50uA
56_1162124863.GIF

1.How about the current "I50A" in this bandgap circuit?
Is it PTAT current or temperature-independent?
I think it is PTAT but I was told that it work as a bandgap current source in a ADC circuit.

2.How about the voltage "Vm" in the middle of the circiut?
Is it PTAT current or temperature-independent?

3. What is the usage of the two resistors "RR" and "RL" in the base terminl of the bipolor transistors?
 

1.How about the current "I50A" in this bandgap circuit?
Is it PTAT current or temperature-independent?
I think it is PTAT but I was told that it work as a bandgap current source in a ADC circuit.
Click to expand...
I50A = 1/R*Vt*ln(m) + Vbe/RL, assuming cascode mirror ratio is=1
where R is the resistor between Vm and the pnp transistor, m is the transistor size ratio of the pnp transistor, Vbe is reffering to the pnp transistor Vbe on the right.
Vt has positive tempco, while Vbe has negative tempco. By having different gain settings 1/R and 1/RL, you may achive a bandgap current reference.


2.How about the voltage "Vm" in the middle of the circiut?
Is it PTAT current or temperature-independent?
Click to expand...
Vm = Vbe of pnp transistor on the right, assuming the opamp offset is negligible.

3. What is the usage of the two resistors "RR" and "RL" in the base terminl of the bipolor FET?
Click to expand...
RR and RL are to generate a current that has negative temp coefficient that can cancel with positive tempco as stated in the I50A equation.
Btw, bipolar and FET is different transistor type. I don think there is a transistor called bipolar FET.
 

    D

    Davidy

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Thank you, sengyee88.

What you analysis is really right.

Do you mind look at another topic of mine about the opa in the bandgap?

That is "What is the usage of the PMOS FET in this simple amplifier?" by me.
 

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