## Wednesday, 23 May 2012

### Feedback loop element identification: Which is which? Non-Inverting Op amp circuit

General Control Loop block diagram:

Given schematic below, how to know which is which? Knowing which is which is critical, as it allows the use of theory to achieve the best result.

Equations that allowed us to identify corresponding blocks:
 From simple voltage divider rule vn = Ri/(Ri + Rf) * vout eq 1 let α = Ri/(Ri+Rf) vn = α * vout eq 2 We see that vout is just opamp gain (Aamp) multiply by its input voltage vout = (vp - vn) * Aamp vout = (vin - α * vout) * Aamp vout * ( 1 + α * Aamp) = vin * Aamp vout / vin = Aamp / ( 1+ α * Aamp) eq 3 From close loop equation of control theory: vout/vin = TF = Aforward/(1 + Aforward*β) eq 4 Comparing eq 3 with eq 4 , we see that Aforward = Aamp Aforward*β = α * Aamp β = α = Ri/(Ri+Rf) When loop gain (Aforward*β) is large enough, we see that 1 + Aforward*β ~= Aforward*β TF = Aforward/ Aforward*β TF = 1/β = (Ri + Rf)/Ri = 1 + (Rf/Ri)

#### 1 comment:

1. Nice blog… Thanks for sharing very useful information about electrical circuits.
Electronic circuit design