NPN BJT Transistor Biasing problem

[Luke Vassallo]

Hi guys,

I am new here. Currently I am working on a school project that requires me to build an "amplifier". Will be using only DC voltage for it's operation.
So far I am using an emitter bias to have negative feedback. My problem is how am I going to know how much voltage I require across the emitter resistor so that I will get a constant 10mA collector current. With that in mind the larger the signal swing the better.

I did some research and some books suggested to use 1/3 of Vcc while others suggested using 1/10 of Vcc, but how much is enough?

 

[BobK]

I would typically use 1/10 of the collector resistor, and bypass it with a capacitor that matches the impedance of the resistor at a frequency of about 1/2 the low end cutoff frequency. The collector resistor would be calculated to drop 1/2 of Vcc at your 10mA bias level.

Edit: Oh, and welcome to the forum!
Bob

 

[davenn]

OK this is school/homework related so have moved the thread

 

[Laplace]

Can't remember ever seeing a stipulation that an amplifier circuit use "only DC voltage for it's operation."

So is this merely stating the obvious, or does it mean something special? Is the amplifier meant to be DC-coupled? Will the input signal be uni-polar?

 

[Luke Vassallo]

I would typically use 1/10 of the collector resistor, and bypass it with a capacitor that matches the impedance of the resistor at a frequency of about 1/2 the low end cutoff frequency. The collector resistor would be calculated to drop 1/2 of Vcc at your 10mA bias level.

Edit: Oh, and welcome to the forum!
Bob

I will not have an ac signal currently, all I am required to do is design the operation of the bjt. I am assuming that by frequency you need an alternating source.

 

[Luke Vassallo]

Can't remember ever seeing a stipulation that an amplifier circuit use "only DC voltage for it's operation."

So is this merely stating the obvious, or does it mean something special? Is the amplifier meant to be DC-coupled? Will the input signal be uni-polar?

The dc voltage I need to set the q point in the middle of the active region so to have the maximum swing possible whilst finding a balance between the swing and stability.

So actually currently this will not have a use in real life.

 

[LvW]

.................
My problem is how am I going to know how much voltage I require across the emitter resistor so that I will get a constant 10mA collector current. With that in mind the larger the signal swing the better.

"You can not have everything you want"

In electronicss, there is one general problem: Everything is a trade-off. That means: Improving one parameter is connected with a degradation of another parameter.
In your case: The "best" signal swing at the output exists without any DC stabilization (Re). Hence, you always have to find the best trade-off between those conflicting requirements: Stability of the operating point vs. signal swing capability.

 

[Luke Vassallo]

Thanks for your replies, I managed to get the BJT working as wanted. I used 24v as my Vcc which I split 4v on the emitter resistor and 10v on the Vce and another 10v on the collector resistor. I got a constant current at the output which varied 0.75mA and 0.5mA on two different transistors, the latter had a higher beta as I tested with the multimeter.

I am actually quite happy with the results as everything was derived using graphs and the equations (Although I got lest variation when I used two values for the potential divider which I got by trial and error) and considering that the variation of 0.75mA was when the transistor was heated up by a soldering iron for 120s.