tranasistor with amplifying factor 200

[ssuet477]

Hi
    Can anyone tell me the name of the transistor whose amplifying factor (beta) is 200. Plz tell me b/c I have to complete my project. thanks

 

[xtp]

If you have a transistor meter you can measure the amplifying factor but it'll be hard to find a low power transistor transistor with that small amplifying factor (you usually get it @ about 350-400 - BC547/BC337 - or at least that's what I measured at aproximately 15 transistors)

 

[Platonas]

What size of transistor do you need?
If it is a normal, small signal transistor, there are many available.  U can use those described in previous post, also have in mind the working voltage, current and power.

To add some more u can use:
BC547C, BC550C, BC548C, BC239C, BC184, BC237B, ZTX108C.

 

[audioguru2]

Hi Ssuet,
Welcome to our forum.  ;D
You can't purchase a transistor with a beta of exactly 200. Their beta come in a range of values for a particular transistor. You can pay more for a BC547B, where the manufacurer has tested and selected a narrow range of beta from 200 to 450, but it changes with temperature, collector current and voltage across it.

Transistor circuits can have negative feedback applied to reduce the range of beta and its changes. Then multiple transistor stages can be added to increase the overall beta to the value you need.  ;D

View attachment 36832

 

[Kevin Weddle]

I'll explain it. The voltage and the current of a transistor dictate the beta. Lower current transistors tend to to have the highest betas. Almost any transistor can give you the desired beta.

Now for the interesting part. There exists what is called the Q point. Simply stated, it is a certain current, normally half the maximum, and a certain voltage across it that gives you the ideal beta area. It says that within this area, you can operate and still achieve an unchanging beta value.

Not only do you want the beta of your choosing, but you also want the beta that is not going to change. You should also opt for lower betas because a lower number means you will get smaller differences in gain.

 

[audioguru2]

Now for the interesting part. There exists what is called the Q point. Simply stated, it is a certain current, normally half the maximum, and a certain voltage across it that gives you the ideal beta area. It says that within this area, you can operate and still achieve an unchanging beta value.

Not only do you want the beta of your choosing, but you also want the beta that is not going to change.

I am sorry but that is not true, Kevin.
In the BC547C example transistor that I posted above, its max collector current is 100mA. If you bias it so that its collector current is half of max, its beta is extremely unlinear. When it is fully conducting 100mA, its beta is typically about 115 from the curve, but will be much less because it will have a very low voltage across it. When it is nearly cutoff at a collector current of maybe 10mA, its beta is typically 280 from the curve, but could be more if its supply voltage is more than 5V.
Even if you bias it at 1/10th of max at 10mA, its beta is not linear.
Also, no matter at what collector current you bias it, its beta will change with temperature.

 

[Kevin Weddle]

What I said about the transistor is all true. But the only thing is that I choose midcurrent as it is a value that rightly falls mid distance between the corresponding maximum values. When they talk about midpoint biasing, they mean the Ic sat. is what your design is and not the data sheet.

It is hard to say that you cannot operate a transistor at half it's current and achieve a reasonably stable beta.

 

[audioguru2]

Kevin,
In addition to a transistor's beta not being constant with collector current and temperature changes, as shown on the datasheets, its transconductance is also non-linear. That is why differential transistor circuits are used as the input stages of audio amplifiers and oscilloscopes, so that the non-linearity of the two differential transistors cancel.

Here is a pic of the output of a single transistor amplifier. Notice its extreme distortion, its positive-going waveform is highly compressed:

View attachment 36837

 

[Kevin Weddle]

I have often wondered if there is a device that would cancel the nonlinearity of the transistor. But then I realized it would then be incorporated into the transistor if that were the case.

 

[audioguru2]

Hi Kevin,
The device to cancel the non-linearity of a transistor is called a differential stage, negative feedback, or an AC-coupled diode circuit like I recently posted in our Therory forum:
http://www.electronics-lab.com/forum/index.php?topic=3057.0