amplificator

[yalect]

Hi, if we consider that the collector voltage is a half DC voltage supply or 3v can you apply my previous propose I mean can I use RC circuit for demodulation without diode ( positve range )may you reread my previous reply ?

[johnsold]

If you changed the bias to get the collector to 3 V, then the output would swing symmetrically until the amplitude exceeded about 2.5 V peak. 

To get detection you need the transistor to conduct for only one half of the input cycle. To do that it must be biased close to saturation or cutoff. The circuit configuration you have is very difficult to bias near either of those points.

Lynn

[yalect]

Hi, john

I have the following circuit I want to connect it to my previous one instead of the transistor but firstly I want to know practically how it can’s work aside I simulated it but I didn’t understand how is it works, if I inject a signal to the base of the transistor with coupling capacitor what is the form of the output signal ? can that transistor amplify the signal or work at one of direction ?

Greeting

[yalect]

Hi, I want to add this comment while you reply for my previous subject

 I think that biasing transistor close to saturation or cutoff will not give much amplification, are there circuits for transistor can do the both ?

[johnsold]

By definition a transistor in cutoff or saturation has very low gain. The size of the signal required to drive the circuit from the low gain operating region to a high gain region is usually fairly large.  Small signal models are not valid in saturation or cutoff (or their paramteers have changed substantially) and large signal models require large signals.

This is why most communications systems have lots of amplification before the detection.

I would suggest using two or three stages of linear amplification with a total overall AC gain of about 1000. Then follow that with a diode detector, the filter, and any DC amplification you might need.

Lynn

[yalect]

Hi, in your reply you said that a transistor in cutoff or saturation has very low gain. The size of the signal required to drive the circuit from the low gain operating region to a high gain region is usually fairly large but when you bias transistor (DC) in one of that region a small size of signal (AC) can do the change.

I found some circuits that use one transistor for detection (conductor for only one half of the input cycle) and amplification but I didn’t how it’s working, might you have an Idea or example for simple one ? in other hand some transistor circuits systems like regenerative have the both function but also it’s working principal not clear because they are not using half cycle conduction method

regards

[johnsold]

1. You have to keep the bias at exactly the right point and there is no good way to determine where that point is as the transistor temperature changes or the power supply drifts some millivolts. Even then the change from one region to another is gradual.

2. If you post an image of the circuit I might be bale to help you understand how it works. Regenerative systems operate on the edge of oscillation (positive feedback) and the signal can move them into and out of oscillation. The sensitivity comes from the positive feedback but that also make them difficult to keep stable.

I do not know of any simple circuit which will do what you want. 

Lynn

[yalect]

Hi, I read that transistor amplifier (class B) could work and conduct half of input cycle only , maybe you have an Idea and I want to know about its gain and if it can be for my circuit ( biasing of the transistor is resistor connected between the collector and the base )

Thank you

[johnsold]

Class B amplifers are typically used in a push-pull configuration with two complementary stages so that both halves of the signal get amplified. They are often used as the output stages of a power amplifier. Some opamps use a modified class B output stage (again with two complementary parts). In those configurations they are typically emitter followers and thus have voltage gain = 1.

There is no way to operate your circuit in class B. The biasing always forces calss A operation.

Lynn

[yalect]

Hello,

I understood that amplifier calss B can’t amplifies small signal like RF but Generally transistor it can’s amplify and conduct half of input cycle only  

thank you