5 ee415  adv electronics  presentation  schwappach

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Transcript of 5 ee415  adv electronics  presentation  schwappach
Feedback InstabilityBy:
Loren Schwappach
Prepared for:
Professor Monica Young
CTU – EE415 – Advanced Electronic Design
September 2010
1
Overview
• Definition of Stability
• Equations
• Design Considerations
• Unstable Design
• Stable Design
• Questions
2
Instability:
In a negative feedback system, a portion of the output signal is subtracted from the input signal to produce an error signal.
However, this error may change as a function of frequency causing subtraction to result in addition.
Negative feedback becomes positive feedback resulting in an unstable system.
3
How to check for stability:
Use a Nyquist frequency plot or verify mathematically.
How to add stability:
•Decrease gain•Add pole•Use compensating capacitor
4
Equations for OPAMP filter design.
𝑉𝑜𝑢𝑡
𝑉𝑖𝑛=
1𝐶𝑠
𝑅 +1𝐶𝑠
=1
1 + 𝑅𝐶𝑠=
1
1 + 𝜏𝑠
1 + 𝐴𝛽 = 1 +𝐾
1 + 𝜏1𝑠 1 + 𝜏2𝑠
𝑠2 + 2𝜁𝜔𝑁𝑠 + 𝜔𝑁2
5
The Design:
Requirements: Design a 3 stage filter for an audio signal (voice components (0 to 4kHz), amplification is preferred but not a requirement.
Design 1:Four pole filter, using noninverting Amps for large gain at cutoff designed as:
Design #2:Drop gain to 1, and add pole to stabilize circuit at 0dB. To find new pole:
𝑓 =1
2 × 𝜋 × 𝑅 × 𝐶→ 𝑅 =
1
2 × 𝜋 × 20𝑘𝐻𝑧 × 1.59𝑢𝐹→ 𝑅 = 5Ω → 𝐶 = 1.59𝑢𝐹
−180° = − tan−1 𝑓−135/𝑓𝑐𝑢𝑡𝑜𝑓𝑓 + tan−1 𝑓−135/𝑓𝑐𝑢𝑡𝑜𝑓𝑓 + tan−1 𝑓−135/𝑓𝑐𝑢𝑡𝑜𝑓𝑓
6
Simple 4 pole LP Filter w/ OPAMP buffer’s (unstable):
Vin
1mVac
0Vdc
Gain1
uA741
+3
2
V+7
V4
OUT6
OS11
OS25
0
0
Buf f er1
uA741
+3
2
V+7
V4
OUT6
OS11
OS25 Buf f er2
uA741
+3
2
V+7
V4
OUT6
OS11
OS25
R4
5 C21.59u
0
Gain4
uA741
+3
2
V+7
V4
OUT6
OS11
OS25
Buf f er3
uA741
+3
2
V+7
V4
OUT6
OS11
OS25
R5
5 C31.59u
R1
1
0
R6
5 C41.59u
0
R2
50
0
Vee
5Vdc
R8100
0
Vcc
5Vdc
R7
1
0
PSpice Schematic  Audio Filter/Amplifer
Designed by Loren K. Schwappach
Designed for Cutoff of 20kHz
Actual Cutoff = 6kHz
R3
5C11.59u
0
7
Simple 4 pole LP Filter w/ OPAMP buffer’s (unstable):
8
Simple 4 pole LP Filter w/ OPAMP buffer’s (unstable): Results
9
Simple 4 pole LP Filter w/ OPAMP buffer’s (unstable): Results
10
Simple 4 pole LP Filter w/ OPAMP buffer’s (unstable): Results
11
Simple 4 pole LP Filter w/ OPAMP buffer’s (unstable): Results
Amp1
uA741
+3
2
V+
7
V4
OUT6
OS11
OS25
R3
5B1
uA741
+3
2V
+7
V4
OUT6
OS11
OS25
C11.59u
R9
70R4
5
B4
uA741
+3
2
V+
7
V4
OUT6
OS11
OS25
C51.59u
B2
uA741
+3
2
V+
7
V4
OUT6
OS11
OS25
C21.59u
0
R5
5B3
uA741
+3
2
V+
7
V4
OUT6
OS11
OS25
C31.59u
R6
5Amp2
uA741
+3
2
V+
7
V4
OUT6
OS11
OS25
C41.59u
R7
1R8
1
0
0
0
0
0
0
0
Vcc
5Vdc
Vee
5Vdc
0
0Vin
1mVac0Vdc
R2
1
R1
1
PSpice Schematic  Audio Filter/Amplifier  Stable
Designed by Loren K. Schwappach
Designed for Cutoff of 20kHz
Actual cutoff 5.689kHz
12
Simple 4 pole LP Filter w/ OPAMP buffer’s (Stable): Results
13
Simple 4 pole LP Filter w/ OPAMP buffer’s (stable): Results
14
Simple 4 pole LP Filter w/ OPAMP buffer’s (stable): Results
15
Conclusions:
• After reducing gain to 1, I was still in an unstable condition forcing me to add another pole.
•By adding another pole I was finally able to stabilize the design.
• If I was going to push forward with this design, I would need to either increase the LP cutoff frequency, recalculate/change the added pole, and add at least 3dB gain.
16
Questions:
17