# Hello, I need help with BJT scheme.

#### krisio

Joined Sep 2, 2014
1
I've attached an image of what my problem is. The resistors are 10k, 1k, 300 ohms, and Vcc is 5volts. I am being asked what will be Uout if Uin is as shown on the characteristic under the scheme.
Any help will be appreciated. Thanks in advance.

#### Jony130

Joined Feb 17, 2009
5,241
First you need to show us your work.

#### dalam

Joined Aug 9, 2014
58
The output will have a 180 degree phase shift as it is Common emitter configuration. Value of beta(Common emitter DC current gain) or hfe is required to find current Ic. Using Ic we can find gm(transconductance). And gain is~= -gm*RL(Load resistance). - sign represents 180 degree phase shift.

#### ScottWang

Joined Aug 23, 2012
7,048
The circuit is an bjt inverting circuit, so the waveform of Vout(not Uout, V is equal to Volt) will be from +5V to 0V as the square wave.

#### dalam

Joined Aug 9, 2014
58
How did you determine Vo without Beta ??
I got Vb=2.273 V and Ib=0.1573mA. Without Beta how did you find Ic?

#### Jony130

Joined Feb 17, 2009
5,241
How did you determine Vo without Beta ??
I got Vb=2.273 V and Ib=0.1573mA. Without Beta how did you find Ic?
Are you sure about that result? And we don't need to know Beta here. Because the BTJ will be saturated.
You can prove this by assuming that Beta is larger than 10.

#### dalam

Joined Aug 9, 2014
58
Are you sure about that result? And we don't need to know Beta here. Because the BTJ will be saturated.
You can do this by assuming that Beta is larger than 10.
Yes I am sure. Vb(max) = Vin(Max)*10/11 and Ib =Vb/10k.
Even if the BJT is saturated then Vo=5-(300*Ic).
And using KVL : -5+300Ic+0.2(Vce sat)=0
This results in Ic= 16mA.
And Vo= Vce sat =0.2V.
Also BJT does not act as amplifier in sat it is just an switch.
Am I right here?

#### Jony130

Joined Feb 17, 2009
5,241

#### Jony130

Joined Feb 17, 2009
5,241
But is my output right ??
Yes, if we assume Vce_sat = 0.2V. Then your answer is right. Vo = Vce_sat for Vin = 2.5V.
But what about the case when Vin = 0V.

#### dalam

Joined Aug 9, 2014
58
Yes, if we assume Vce_sat = 0.2V. Then your answer is right. Vo = Vce_sat for Vin = 2.5V.
But what about the case when Vin = 0V.
Well in that case the BJT enters into Cut-off mode and it is an open circuit which implies Vo= 5V.