# Voltage Divider with a Transistor Help

Discussion in 'General Electronics Chat' started by rachshot65, Feb 2, 2010.

1. ### rachshot65 Thread Starter New Member

Oct 25, 2008
7
0
Hi,

I'm currently listening to a podcast on building a computer from fundamentals and they say this:

Now then I think I understand the first two examples and I made some circuit diagrams:

and then with the resistor replaced with a switch

But I confused about the transistor example. I tried to make a diagram of it but I'm not sure about the connections of the legs:

Could anyone explain how replacing the switch with a transistor works and maybe correct my diagram ?

Thanks

2. ### Jony130 AAC Fanatic!

Feb 17, 2009
3,993
1,116
Connect base trough 10K resistor to 9V power supply.

3. ### jcbeck84 Active Member

Dec 20, 2008
46
0
Wouldn't this eventually settle so that the voltage at the collector is at some point between 0V and 9V? or would the diode junction in the transister be enough to stop this small current from flowing?

4. ### rachshot65 Thread Starter New Member

Oct 25, 2008
7
0
Hi,

Thanks for the reply I have connected the transistor as you suggested

Is this correct ? Is there a reason it's 629mV and not 0v ?

5. ### SgtWookie Expert

Jul 17, 2007
22,183
1,728
There will be some small leakage current through the transistor when there is no current flow in the base, but it's a very small amount of current.

In the podcast, the narrator talks about "voltage flow". This is not correct. Current flows; voltage is pressure. Current flow through resistance causes a voltage "drop" across the resistance.

He mentions conventional flow vs electron flow. Conventional flow was thought up by Benjamin Franklin many years ago. In conventional flow, current goes from more positive to more negative. In electron flow, the electrons go from more negative to more positive.

When you are using a transistor as a switch, the generally accepted standard for transistor saturation is that the base current flow should be 1/10 the desired collector current.

So, Rb =(V-Vbe) / (Ic/10)
where:
Rb = Base current limiting resistor value in Ohms
V = the voltage supply for the base current
Vbe = the voltage at the base measured in respect to the emitter when the desired base current is flowing. You can usually start with 0.7v for this value; it will change if base current is very low or very high.
Ic = collector current in Amperes

So, your collector resistor is 10k Ohms, and Vcc=9v. Since I=E/R, 9v/10k Ohms = 0.9mA.
Back to the formula:
Rb =(V-Vbe) / (Ic/10)
Let's plug in Ic, V and Vbe:
Rb = (9V-0.7V) / (0.9mA/10)
And work the problem:
Rb = 8.3V / 0.000,09 A
Rb = 92.222k Ohms.
That is not a standard value of resistance. A standard chart is here:
http://www.logwell.com/tech/components/resistor_values.html
Bookmark that page.

Looking at the E24 values (green columns) we can see that you could use 91K Ohms, as that is a standard value. In this case, you could also likely use 100k Ohms. That is a little less base current than the standard rule, but it would likely be close enough to work.

[eta]
Here is the circuit with R2 added as the base resistor. You can see that the Vce (voltage from collector to emitter) is very low now.

• ###### Transistor as a switch.PNG
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Last edited: Feb 2, 2010
6. ### SgtWookie Expert

Jul 17, 2007
22,183
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That's not exactly what he was thinking of, but I understand why you thought you could connect it that way.
No.
Yes.
As the current from your 10k resistor turns on the transistor, the collector sinks current. The Vce settles at the Vbe for the given current flow; in this case, 0.619v. Note that the simulation won't be completely accurate; what you get with actual components may vary. The BE junction acts like a diode that is forward biased.

7. ### rachshot65 Thread Starter New Member

Oct 25, 2008
7
0
Hi,

Thanks for all your replies and help

So is this called a called and inverter as you get ~ 0v out from a positive voltage in ?

Thanks

8. ### rachshot65 Thread Starter New Member

Oct 25, 2008
7
0
Hi,

So I continued listening now I understand that and he moved on to talk about using two transistors in parallel to create a NOR gate

Is this diagram a correct representation of this ?

Thanks

Last edited: Feb 3, 2010
9. ### Jony130 AAC Fanatic!

Feb 17, 2009
3,993
1,116
Look at example diagrams

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