why transistors in preamp

[asimkumar]

i have seen most of the rotating laser receivers(used for grading or leveling) use transistors in preamp section. why do they choose transistors instead of opamp. i don't think they bother about cost and input bias current is not important because there are some op amps with pico amp bias current,so why they use transistors.

i cant upload a circuit, a rotating laser plane(horizontal) detected by a vertical array of photo diodes, signal from the diodes is amplified and detected by a microprocessor, photo diodes usually in reverse bias for high sensitivity, these receivers work with range of 1000 meter with only 3milli watt red laser beam.

 

[FvM]

i cant upload a circuit

Is it so? Well there can't be a serious answer without knowing at least qualitative parameters of the circuit under discussion.

I can imagine many reasons why the the designer chosed transistors for the input stage. E.g. power consumption, form factor, noise, costs, component quotation, tradition and also - technical ignorance. If you are trying to design a new instrument by reverse engineering existing products, the exact answer won't be really important - once you understand the technical requirements. You can chose your own way, and possibly design a better instrument or re-invent the square wheel, depending on your knowledge.

 

[asimkumar]

Yes u guessed right am trying to reverse engineer, u can see some equivalent circuits

**broken link removed**

**broken link removed**

 

[FvM]

I would primarly refer to the description and claims to understand the said circuit's purpose. A common base amplifier with or without succeeding stages can be understood as a TIA equivalent. Referring to state-of-the-art for other demanding TIA applications, I expect that usual OP TIA designs can replace it with same or better performance.

You'll find out if you start to calculate actual application parameters.

 

[chuckey]

This circuit relies on the input tuned circuit to reject noise. The designer chose to use the very low input impedance of the common base transistor as a load for his tuned circuit. If he had used a CE amp , he would have had a higher input impedance to work into so he would have needed a bigger inductance/smaller cap to get the same characteristics.
Frank