Extra Notes for Lecture 07

Extra Notes for Lecture 07 - Typically we do this at the...

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Derivation of input and output resistances for 5V b 4V Step-Down Converter Input Resistance: 1. The most important step is to find where to calculate the input resistance from. Here we have a 50 ohm source resistance that is associated with the source only. The input resistance is associated with the voltage Vin. 2. Finding the input resistance is pretty easy here. Remember we only look forward, so ignore the source. We first hit the 50 ohm resistor, then a parallel combination of 400 ohm and the load. 3. Therefore g = 50 Ω + (400 Ω||g ² ) 4. If there is no load, g = 450 Ω . This is the max value of g ! As g ² decreases, g will drop to a minimum of 50 ohms. What does this mean for Vin? Output Resistance: 1. Again, the most important step is to find where to calculate the output resistance from.
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Unformatted text preview: Typically we do this at the node that the load is connected to. The load is associated with the voltage Vout. 2. We need to follow Thévenin / superposition rules when it comes to independent sources. We short the 5 V source and then calculate the equivalent resistance. We see two parallel paths to ground: 400 ohms and 2 50 ohm resistors in series. 3. Therefore g ³´µ = (50 Ω + 50 Ω)||400 Ω = 80 Ω 4. If there is no load, Vout will equal 4 V. This is the max value of Vout! What happens to Vout as g ² decreases? From here it can be seen the ideal load is no load! Or a very large load.Unfortunately this is an unavoidable problem....
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This note was uploaded on 09/09/2011 for the course EECS 40 taught by Professor Chang-hasnain during the Summer '08 term at Berkeley.

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