# Transistors

Discussion in 'Embedded Systems and Microcontrollers' started by iPromise, Aug 11, 2013.

1. ### iPromise Thread Starter New Member

Aug 11, 2013
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I read that Transistors amplify the voltage from input.

But how does amplifying voltage help create logic gates preforming operations (AND gate, OR gate, XOR gate, NAND, etc.)?

2. ### tracecom AAC Fanatic!

Apr 16, 2010
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Transistors also function as switches, and it is in that mode that they may be arranged as logic gates. See two examples attached.

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3. ### iPromise Thread Starter New Member

Aug 11, 2013
15
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Thanks for the quick and clear answer!

I'm beginning to love this forum.

Jul 18, 2013
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A bi-polar transistor is considered a current amplifier, a FET could be considered a voltage amplifier.
Max.

5. ### onlyvinod56 Active Member

Oct 14, 2008
362
1
consider an npn CE mode. if a relay coil is connected between collector and Vcc.

the collector current Ic = (Vcc - Vce)/R.relay.
How can the BJT amplifies the current as already the current is decided by the relay coil resistance??

6. ### ScottWang Moderator

Aug 23, 2012
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The normally BJT amplifies the current is working in the linear area, the Ic is depends on the Ib and count on hfe=Ic/Ib, and as you said to used the relay that is working in the logical area, hfe=Ic/Ib=10 for Vce saturation.

7. ### jegues Well-Known Member

Sep 13, 2010
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BJTs are often described as both a current controlled and voltage controlled current sources, but under varying circumstances.

The current control model, $I_{C} = h_{FE}I_{B}$ is often used when we are in the linear region of operation, as this is "good enough."

However, other models like the Eber-Molls model, $I_{C} = I_{S} \left(e^{\frac{V_{BE}}{V_{T}}}-1 \right)$ are used to address the fact that transistors are indeed transconductance devices, meaning that the collector current is determined by its base-to-emitter voltage.

Last edited: Nov 14, 2013