| ??? 01/13/06 04:30 Read: times |
#107233 - JFET Input good Responding to: ???'s previous message |
This circuit is a simple Source-follower with a current source as source resistance. Source drain current is about 5mA, guaranteeing a rather low output impedance (in the 100Ohm range). So, a load impedance of 1MOhm at the FET-buffer output would only result in a gain error of less than 0.1%, which is totally negligible for a scope application. So, the AD844 is well suited to buffer the FET-buffer. Dear Kai, Thanks for the simple and complete explanation.I updated the Input section of the schematic I've posted ,Please use the Below link and do not forget to press Shift+Refresh. http://www.8052.com/users/far...pdated.GIF There are not many circuits being simpler that this Source-follower! Again, this circuit presents no gain but 1, so what would you expect to degrade? No, this circuit is highly stable and is widely used in all sectors of electronics. It benefits from the brilliant HF performance of FET. Actually , after seeing these high speed Op-Amps I couldn't believe such an inexpensive Circuit using discretes could do such a wonderful job, But as you say it does. In combination with a 1:1/10:1 probe you can handle an input voltage range of up to 500V. Thanks to the great 3 level division input suggested by you all of these are possible. The gain numbers you've typed are just the same that I carried out in paper based on your posted schematic in the last reply. Best Regards, Farshid |
