QA 605

Low noise monolithic circuits QA 605 generally have quite high current noise densities, rendering them unsuitable for source impedances (real value) of over 250, not to mention impedances of 1k or higher. Bias current of the input differential pair in a low noise amplifier has to be relatively high to reduce input noise voltage density. In the case of a bipolar input stage, this would cause the input bias current to rise beyond acceptable levels. Therefore designers often provided the input stage with current sources - which make the input current noise density even worse and may cause instabilities for higher source impedances.

This hybrid design shows none of the above mentioned drawbacks: Current noise density is low even with excellent voltage noise density, the amplifier has low input bias current and will not become unstable for high source impedances.

The circuit topology is internally differential, but with single ended input and output. There are generally no external components required to run the amplifier, supply bypass capacitors are integrated. However, if you intend to run the amplifier with high output load, additional bypass capacitors (tantalum) are recommended. The amplifier fits into DIL-14 sockets and features a hermetically closed (welded) metal package which provides RFI-protection and high reliability even under difficult environmental conditons.

The amplifier QA 605 is recommended for ultrasonic signal conditioning, especially for transducer
impedances in the 1k range and frequencies up to 1.5MHz. By its outstanding DC input
characteristics the amplifier also qualifies for low noise instrumentation or sensor
applications.