I'm trying to understand this schematic of this differential probe:
I marked some blocks. I have some questions about the block I marked "Long-tailed pair" (LTP). I have no experience with LTPs so excuse ma noob questions.
- I have simulated this LTP (just the LTP without all the other blocks in the schematic) in LTspice and it's bandwidth is around 26MHz. How to increase it's bandwidth (beyond 100MHz)? What are the main limitations here in this LTP?
- What is the purpose of BJTs (U2B, U2C) there? The only thing I could think about it is that it offloads the JFETs from the current. Is that right? Or is there another reason for that? When I remove BJTs (U2B, U2C, and their resistors R8, R9) then bandwidth increases to 52MHz. So I'm curious what is the purpose of them, because they alone halves the bandwidth.
- I understand that R12 and R13 are emitter degeneration resistors (or better say "source degeneration"?). Anyway, how to determine overall gain of such long-tailed pair?
- What is the output resistance/impedance of this LTP? Is it R14, R16 or is it R12, R13?
Now as I see these charts, it is weird. I considered it is a 26MHz bandwidth, but it looks like a high-pass filter. Because lower frequencies are at -13dB, and higher frequencies are at 0dB.