Why use discrete components for preamplification?

飞絮

Why use discrete components for preamplification? [Copy link]

I recently surveyed an instrument and found that the preamplifier circuit was composed of discrete components 3DJ6 and 9012, which was used to amplify the uV-level sine wave. The output of this amplifier circuit entered the op amp. I couldn't understand why the amplifier circuit with discrete components was better than the op amp? Why not use the op amp directly as the preamplifier circuit? What is the principle of this preamplifier circuit?

annysky2012

Some integrated ones cannot meet your design requirements, so it is better to use separate ones.

chunyang

The original poster's circuit takes advantage of the high input impedance of the junction field effect tube. If you use an op amp directly, you need to use a model with a junction field effect tube as the input stage. Compared with conventional models, such op amp models are much less and more expensive, especially in the era of 3DJ6. 3DJ6 was a popular domestic tube in the 1980s. After entering the 1990s, it was gradually used by fewer and fewer people. I am afraid it has been discontinued for many years. When 3DJ6 was used to design circuits, op amps were still very expensive and difficult to buy, especially models with high input impedance. In this way, in applications that require high input impedance, it is normal to use a design with a front-end discrete junction field effect tube input. Of course, similar new applications should be seen in the future, because there is the word "copycat".

cruelfox

"Separate components", where did you learn this?

maychang

Early high input impedance op amps were very expensive and hard to find, so discrete components were used. If you still use discrete components today, it is considered a copycat circuit or board.

飞絮

I understand. In designs where price is not a concern, try to use op amps. This circuit was designed early, more than 20 years ago.

chunyang

Feixu published on 2021-5-17 16:56 I understand. In designs where price is not a concern, try to use op amps. This circuit was designed in the early days, more than 20 years ago.

Now, directly using an op amp for the junction field-effect transistor input stage may not necessarily be more expensive than adding a separate field-effect transistor to the design, and ready-made op amps should be used.