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#1
04-29-2012, 06:36 AM
 theertham Junior Member Join Date: Apr 2012 Posts: 26
transistor as an invertor circuit

can any body tell me the working of transistor as an invertor circuit.figure is attached
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#2
04-29-2012, 06:49 AM
 blah2222 Senior Member Join Date: May 2010 Location: West Posts: 506

Quote:
 Originally Posted by theertham can any body tell me the working of transistor as an invertor circuit.figure is attached
Well for AC-small-signal operation (DC = 0) you can use this model of the BJT to figure out what's going to happen:

In your case, you have the input connected to the base via R9 (10k), emitter tied to ground, and R10 (10k) in parallel with the output resistance (rout) of the BJT.

You need to find Vout as a function of Vin by first finding Vbe as a function of Vin and then Vout as a function of Vbe:

$V_{be} = \frac{r_{\pi}}{r_{\pi} + R_{9}}V_{in}$

Knowing this, you can find out the amount of small-signal current running through the current source. Since the emitter is grounded, the current through this source will run through the parallel contribution of rout and R10 from ground to Vout. With this in mind, we can setup the following KCL equation:

$g_{m}V_{be} = \frac{0 - V_{out}}{r_{out}||R_{10}}$

Rearranging and subbing in for Vbe:

$\frac{V_{out}}{V_{in}} = -g_{m}(\frac{r_{\pi}}{r_{\pi} + R_{9}})(r_{out}||R_{10})V_{in}$

Where:

$g_{m} ~= 40I_{C}$

$r_{\pi} = \frac{\beta}{g_{m}}$

$r_{out} = \frac{V_{A} + V_{CE}}{I_{C}}$

The negative sign shows why this is an inverting amplifier. (Math approach)

Last edited by blah2222; 04-29-2012 at 07:33 AM.
 The Following User Says Thank You to blah2222 For This Useful Post: theertham (04-29-2012)
#3
04-29-2012, 07:22 AM
 crutschow Senior Member Join Date: Mar 2008 Location: L.A. USA Posts: 6,304 Blog Entries: 1

Its fairly simple.

Start out with zero volts on the input. This means the transistor is off and the collector output is at the supply voltage since there is no current through the collector resistor, R10.

Now start increasing the input voltage. That will cause base current to flow through R9 and the transistor will start to turn on with current starting to flow through R10. The causes an IR voltage drop across R10, reducing the collector voltage.

Thus as the input voltage is increasing, the output voltage is being reduced, thus the circuit acts as an inverter.
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 The Following User Says Thank You to crutschow For This Useful Post: theertham (04-29-2012)
#4
04-30-2012, 01:24 PM
 theertham Junior Member Join Date: Apr 2012 Posts: 26
transistor as inverting circuit

hi
i have attached the output from transistor inverter circuit which has 10k at the collector and 1 k at the base with emitter grounded.the output shows there is a slight curve at the top of the pulse.but when i connected 10k at the base and 1k at the collector with emitter ground,the output became perfect pulse.what could be the reason?
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 2012-04-20 11.55.06.jpg (37.5 KB, 13 views)
#5
05-01-2012, 07:38 AM
 theertham Junior Member Join Date: Apr 2012 Posts: 26
transistor inverter circuit

[QUOTE=theertham;482160]hi
i have attached the output from transistor inverter circuit which has 10k at the collector and 1 k at the base with emitter grounded.the output shows there is a slight curve at the top of the pulse.but when i connected 10k at the base and 1k at the collector with emitter ground,the output became perfect pulse.what could be the reason?[/important]
#6
05-01-2012, 07:49 AM
 Jony130 Senior Member Join Date: Feb 2009 Location: Poland/Wroclaw Posts: 2,456

The effect you describe is caused by the parasitic capacity.
This capacitor is connect between collector and the gnd.

 Tags circuit, invertor, transistor

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