RC Circuit

[nicksydney]

Hi,

I'm in the middle of learning about RC Circuit and one of the thing I'm learning is about Amplifier Circuit. I'm reading through this website and at the moment going through the 'The Common Emitter Amplifier Circuit' section. I have few questions based on the simulation from this website

1. It is explained in that website that "The capacitor and the 110k and 10k resistors bias the transistor's base at about 1.7 V" I understand how to calculate resistors in series but I can't get the value of 1.7v, how to do this
calculation ?

2. Why do we need to put the 1K resistor at the emitter going to ground ?. I can understand the need and use of resistor near the input but why near ground ?

3. Capacitor is used mostly for storage of enery and in the case of this circuit I believe that the 5uF capacitor used is to make sure that the output is stable (no spike) but I failed to understand why we need to use the
1M resistor near the capacitor ?

Thanks for your kind help

 

[ericgibbs]

Hi,
1. It is explained in that website that "The capacitor and the 110k and 10k resistors bias the transistor's base at about 1.7 V" I understand how to calculate resistors in series but I can't get the value of 1.7v, how to do this
calculation ?

The value of Vb = Vsupply * [10k/(10k+110k)] = 1.66V [ its a ratio of the two series resistors]


2. Why do we need to put the 1K resistor at the emitter going to ground ?. I can understand the need and use of resistor near the input but why near ground ?

The 1K is emitter feedback, with a 10K collector resistor the Gain of the Stage is ~ 10K/1K =10

3. Capacitor is used mostly for storage of enery and in the case of this circuit I believe that the 5uF capacitor used is to make sure that the output is stable (no spike)

On the Collector of the transistor is a DC voltage, which needs to be blocked from appearing across the Load [ the 1meg resistor] . The 5uF only allows the AC signal thru to the 1meg


but I failed to understand why we need to use the
1M resistor near the capacitor ?
Its just a simulated load

Thanks for your kind help

hi,
Hope this helps.

 

[Winterstone]

2. Why do we need to put the 1K resistor at the emitter going to ground ?. I can understand the need and use of resistor near the input but why near ground ?

Hi Nicksydney,

perhaps it helps to say something more to the emitter resistor Re.
As mentioned before, it provides a certain amount of voltage feeedback, which in most cases is necessary to stabilize the bias point of the BJT against (a) tolerances of the current gain (sometimes up to 100%) and (b) even more important: collector current variations due to temperature changes (external or caused by the BJT itself).
The principle how such a resistor Re can provide feedback deserves some additional explanations.
My recommendation: Try to understand the fundamental principle of feedback because it is applied in nearly all amplification stages. In this context, it is important to discriminate between dc and ac feedback - and its consequences on gain and frequency properties of the amplifier.
Good luck

 

[nicksydney]

Hi Winterstone,ericgibbs

The feedback that you are referring to is it the same as outlined in this

Thanks

 

[Lund_Pa]

The 1K prevents thermal runaway and keeps the device from burning up.

 

[Winterstone]

Hi Winterstone,ericgibbs
The feedback that you are referring to is it the same as outlined in this
Thanks

Yes, of course. It is one of the classical methods for stabilization of the operating point.
BTW: The other method (voltage controlled current feedback) is also shown on the same page.

 

[nicksydney]

Thanks all for the reply.