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A differential amplifier, basically, consists in two input stage, that amplifies the difference between those inputs. Actually, differential amplifiers are made by a chain of stages that work amplifiying and controlling signal and bias.
Youmay read these documents for get a general view:
RF Microelectronics Book By Razavi **broken link removed**
You want to make a pushpull amplifier with a low current transistor on the input. You can make a pushpull that will respond to the signal when properly biased. All you need is the two pushpull transistors and a bias transistor that will phase split the voltage. It's the low current transistor that makes the inpedance high.
If you can usse three operational amplifier in order to get more input impedance, you may built an instrumentation amplifier. Also, you may get high CMRR.
If I understand op, I must build feedback with resistors to get some concrete amplifaction factor. This feedback circuit will decrease input impedance and FET inputs will not help with this. Without feedback it is more comparator than differential amplifier.
I want to build a simple amplifier for Hall senzor, the smallest possible becouse I want to place it into lpg multiventil.
8) I had seen an application note from "Linear technology" in EDN which used three opamp and a referance for Hall sensor .i think u can serach there web site.
If I understand op, I must build feedback with resistors to get some concrete amplifaction factor. This feedback circuit will decrease input impedance and FET inputs will not help with this. Without feedback it is more comparator than differential amplifier.
I want to build a simple amplifier for Hall senzor, the smallest possible becouse I want to place it into lpg multiventil.
The feeback resistors are the trick to the opamp. The design is such that because of the inversion within the opamp, it will tend to work output to inverting input. I would stick to a pushpull that is biased with another transistor, called the phase splitter, and limit the gain with a collector and emitter resistor. I can't show you the circuit, but if you want to design the basic opamp then you will have to do a bit of studying. The pushpull are two NPN's arranged like a totem pole. If you look at the schematic for an opamp, the ouput is buffered and you have a NPN with a PNP arranged like a totem pole. This is just a voltage follower.
Instrumentation amplifier is a good choice since they use non-inverting inputs as the input terminals. Generally the feedback resistors are connected to the inverting inputs. IN such a way you don't have to worry about the impedance caused by the feedback resistors since you're using non-inverting inputs in instrumentation amplifier.
You must use MOS amplifiers to get high input resistance. for example you can use telescopy, folded cascode amplifier with the second stage like push pull stage to get high gain. you can study the Analog integration circuits written by Razavi, Ken martin, temes.
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