The OP07 opamp has an BJT input. Typically this means a lower input impedance/resistance. Why is this not the case for the OP07? The reason I ask is that I was looking at the ProCo Rat schematic and was curious as to why it's typically listed as a 1M/500k input impedance circuit when it has an BJT opamp at its input stage.
Thanks in advance for any help 🙂
Thanks in advance for any help 🙂
OP07 isn't an outlier in terms of Zin (input impedance) for a bipolar input device. Its just that FET input opamps have even higher Zins.
The lowest Zin normally correlates with lowest input noise as lowest noise calls for physically large devices running at appreciable collector current. LT1028 (a very low noise device) for example has a differential mode Zin of 20k, that's pretty low. But in a non-inverting configuration that impedance is heavily bootstrapped by the NFB (negative feedback). Meaning you'll never see 20k or anywhere near it in practice. You will see the CM (common-mode) Zin, but that's extremely high at 300M. Admittedly, OP07's 160G is orders of magnitude higher still. AD797 (another very low noise opamp) has even lower Zin figures than LT1028 but they're rarely an issue in practice.
The lowest Zin normally correlates with lowest input noise as lowest noise calls for physically large devices running at appreciable collector current. LT1028 (a very low noise device) for example has a differential mode Zin of 20k, that's pretty low. But in a non-inverting configuration that impedance is heavily bootstrapped by the NFB (negative feedback). Meaning you'll never see 20k or anywhere near it in practice. You will see the CM (common-mode) Zin, but that's extremely high at 300M. Admittedly, OP07's 160G is orders of magnitude higher still. AD797 (another very low noise opamp) has even lower Zin figures than LT1028 but they're rarely an issue in practice.
If you look at the input stage topology in the datasheet you'll see its fed from constant current from each rail.
When you look at the internal schematic in the Analog Devices datasheet, you see that each input transistor has its own base current compensation circuit. As a result, the base current compensation circuit also works well when there is a differential voltage at the input. Most op-amps with base current compensation only correct the common-mode current, that is, only the base current when the input stage is in balance.
Besides, as abraxalito already mentioned, the tail current of the input stage must be relatively low. You can infer that from the noise specifications: relatively high equivalent input noise voltage density, relatively low equivalent input noise current density. (Large tail currents reduce noise voltage and increase noise current.)
Besides, as abraxalito already mentioned, the tail current of the input stage must be relatively low. You can infer that from the noise specifications: relatively high equivalent input noise voltage density, relatively low equivalent input noise current density. (Large tail currents reduce noise voltage and increase noise current.)
Negative feedback applied to the opamp will also drastically raise the input impedance of the circuit.
Tom
Tom
All in all, using input series feedback, you can make the input impedance extremely high while using a bipolar input stage. Low tail currents and certain kinds of base current compensation also help.
What you cannot do with a bipolar input stage is to make the input noise current extremely low without also getting a high input noise voltage. That's why amplifiers that have to have low noise and work with a very-high-impedance source, such as the preamplifiers built into condenser microphones, never have a bipolar input stage.