2.3 The Non-Inverting Configuration

Transcription

1 2/18/2011 section 2_3 The non inverting configuration 1/1 2.3 The NonInverting Configuration Reading Assignment: pp. Another standard opamp circuit configuration is the noninverting configuration. HO: THE NONINVERTING CONFIGURATION An important noninverting circuit is the voltage vollower. HO: THE VOLTAGE FOLLOWER

2 2/18/2011 The NonInverting Configuration lecture 1/4 The NonInverting Configuration R 1 R 2 i 1 i 2 i =0 v i =0 v ideal Good heavens! The inverting input (v ) of this configuration is not at virtual ground (i.e., v 0 )! Recall that v =v (the virtual short) ALWAYS for feedback amplifiers.

3 2/18/2011 The NonInverting Configuration lecture 2/4 No virtual ground here! Notice also that for the circuit above, the voltage at the noninverting terminal is the input voltage : v = 0 We use this fact to analyze this noninverting configuration. First, we use KCL to determine that: R 1 R 2 i = i i 1 2 i 1 i 2 and since i = 0, we again find that: and from Ohm s Law: i = i 1 2 i =0 i =0 v v ideal 0 v v v vout 1 = = 2 i = R1 R1 R1 i

4 2/18/2011 The NonInverting Configuration lecture 3/4 i = 0 is the key These results are of course very similar to the expressions we derived when analyzing the inverting configuration. The main difference is of course that v is not equal to zero. Instead, we know that v = v in. Thus: v v v i = i = in in out 1 2 R1 R1 and since 1 i = 2 i, we determine a relationship involving and only: v v v = R R in in out 1 2 R 1 R 2 i 1 i 2 i =0 i =0 v v ideal

5 2/18/2011 The NonInverting Configuration lecture 4/4 Note the gain is a positive number Performing some simple algebra, we rearrange this expression and find the opencircuit voltage gain of the noninverting configuration: oc vout R2 Avo = = 1 v R in 1 Note that the opencircuit voltage gain for this configuration is a positive number. We conclude then that the input and output voltage will have the same sign (i.e.,± ). This is why we call the configuration noninverting.

6 2/18/2011 The Voltage Follower lecture 1/7 The Voltage Follower The voltage follower has a opencircuit voltage gain A vo = 1 with the result that v = v! R out in ideal v out = v in Q: Pfft! The output voltage is equal to the input voltage?! Why even bother? A: To see why the voltage follower is important, consider the following example.

7 2/18/2011 The Voltage Follower lecture 2/7 What a great amp Say you have toiled for hours to design and build the following audio amplifier: 15V 15V A vo = 200 (midband) R out = 1 K 3.7K 1K 0.1 μf R i = 370 Ω 0.1 μf 2.3K 1K 0.1 μf Q: A vo = 200!With this much gain we ll be shakin the windows right?

8 2/18/2011 The Voltage Follower lecture 3/7 or, maybe not A: Actually, if we connected this amplifier directly to a speaker, nothing would happen silence! Q:??? A: The reason for this is that the resistance of most speakers is very small (4 Ω8 Ω).

9 2/18/2011 The Voltage Follower lecture 4/7 What s the problem then? We can use the linear equivalent circuit model of the audio amplifier to analyze the result: 370Ω 1K 200v in 4Ω 4 v 200v = = 0. 8v out in in The output of this amplifier is even smaller than its input! The problem, of course, is not that the opencircuit voltage gain is too small after all, it s 200!

10 2/18/2011 The Voltage Follower lecture 5/7 The output resistance is just too large! The problem is that the amplifier output resistance (R out = 377Ω) is much larger than the load resistance R L = 4 Ω. Therefore, we have tremendous loss due to the resulting voltage divider: V 15V R There is a solution to this problem use a voltage follower! 3.7K 1K 0.1 μf 4Ω 0.1 μf 2.3K 1K 0.1 μf

11 2/18/2011 The Voltage Follower lecture 6/7 The voltage follower to the rescue! Let s again use the linear equivalent model to analyze this circuit and find the output voltage. 1K 370Ω 200v i v i1 1v i 1 4Ω We ve got back our gain! 4 v = 200v 1 = 200v out in in

12 2/18/2011 The Voltage Follower lecture 7/7 The voltage follower: a useful buffer Note: 1. Instead of 4Ω, the audio amp sees a load of, the input resistance of the voltage follower this is ideal! 2. Instead of 377Ω, the speaker sees a source resistance of 0, the output resistance of the voltage follower this too is ideal! Remember, there are three characterizing parameters of an amplifier open circuit voltage gain is just one of those three! The input and output impedance of the voltage follower make it an excellent buffer between two circuits!