Chip manual calculation problem

S3S4S5S6

Chip manual calculation problem [Copy link]

Vout calculation Vout=2V+Ipd*RF; How is this 2V calculated?

maychang

When the op amp is in the linear working area, it always tries to make the potential of the two input terminals equal, which is called "virtual short", so the op amp inverting input terminal is also 2.5V. BF862 is a junction field effect transistor, and its gate is slightly lower than the source in normal operation (how much lower depends on the source current, see the junction field effect transistor output characteristic curve), so the op amp output is 2V plus the voltage across Rf.

S3S4S5S6

maychang published on 2019-4-16 16:42 When the op amp is in the linear working area, it always tries to make the potential of the two input terminals equal, that is, the so-called "virtual short", so the op amp inverting input terminal is also 2.5V. BF862 is...

Another question is, what is the role of this tube here, why do we need this tube, the conventional circuit is followed by a megohm resistor, and I checked the input resistance of this op amp and it is several hundred K ohms, does the tube here play the role of increasing the input impedance? Upload a manual, please help me take a look

maychang

S3S4S5S6 Published on 2019-4-16 17:05 Another question is, what is the role of this tube here and why is this tube needed? Conventional circuits all have a megohm resistor behind the PD...

"The input resistance of this op amp is several hundred K ohms. Does the tube here serve to increase the input impedance?" I think the purpose of adding a junction field-effect transistor is to increase the input impedance.

maychang

S3S4S5S6 Published on 2019-4-16 17:05 Another question is, what is the role of this tube here, why is this tube needed, the conventional circuit is followed by a megohm resistor...

I don't know why this type of op amp is used. This op amp is estimated to be expensive. If the frequency response requirements of the phototube are not very high, you can consider using other types of op amps.