Prev: When and Why use resistors between base and emitter
Next: Phase shift in cabling?
From: neddie on 3 Jun 2010 10:02
Hi to all.
I've put together( well simulated :0) ) a discrete LDO cct using an
opamp(lm358) and a p-channel mosfet.
Under certain conditions it oscillates. How do I go about compensating
this circuit so that it does not oscillate.
I must stress I'm not going to actually build this circuit , I'm just
playing around.
If R9 (0.01) ohms is increaced to about 0.1 ohms or higher the circuit
oscillates.Is it even possible to
fix this , or is the circuit inherently unstable.As I aid I'm not
going to build it ,I can buy a LDO :0) , I'm just playing around.The
left part of the cct is just a constant current source feeding a zener
for the voltage reference.
The cct is in LTSpice format. LM358 model included.Just copy and put
into -> file.asc

Cheers
Rob

Version 4
SHEET 1 3536 1904
WIRE -112 -256 -208 -256
WIRE -64 -256 -112 -256
WIRE 160 -256 -64 -256
WIRE 288 -256 160 -256
WIRE 448 -256 384 -256
WIRE 528 -256 448 -256
WIRE 608 -256 528 -256
WIRE -208 -224 -208 -256
WIRE -112 -224 -112 -256
WIRE 448 -160 448 -256
WIRE -112 -128 -112 -144
WIRE 304 -128 304 -208
WIRE -208 -80 -208 -160
WIRE -176 -80 -208 -80
WIRE 128 -80 80 -80
WIRE 256 -80 208 -80
WIRE -208 -48 -208 -80
WIRE 608 -48 608 -256
WIRE 608 -16 608 -48
WIRE -464 0 -464 -48
WIRE 528 0 528 -256
WIRE 448 16 448 -80
WIRE 448 16 352 16
WIRE -208 48 -208 32
WIRE 160 48 160 -256
WIRE -112 64 -112 -32
WIRE -48 64 -112 64
WIRE 80 64 80 -80
WIRE 80 64 32 64
WIRE 128 64 80 64
WIRE 256 80 256 -80
WIRE 256 80 192 80
WIRE 304 80 304 -48
WIRE 304 80 256 80
WIRE 448 80 448 16
WIRE -464 96 -464 80
WIRE 128 96 32 96
WIRE -112 112 -112 64
WIRE -112 112 -208 112
WIRE -208 144 -208 112
WIRE -112 160 -112 112
WIRE 32 176 32 96
WIRE 352 176 352 16
WIRE 352 176 32 176
WIRE -208 288 -208 208
WIRE -112 288 -112 224
WIRE -112 288 -208 288
WIRE 160 288 160 112
WIRE 160 288 -112 288
WIRE 288 288 160 288
WIRE 448 288 448 160
WIRE 448 288 368 288
WIRE 528 288 528 64
WIRE 528 288 448 288
WIRE 608 288 608 64
WIRE 608 288 528 288
WIRE 160 304 160 288
FLAG 160 304 0
FLAG -464 96 0
FLAG -464 -48 SUPPLY
FLAG -208 48 0
FLAG 608 -48 Load
FLAG -64 -256 SUPPLY
SYMBOL voltage -464 -16 R0
WINDOW 123 0 0 Left 0
WINDOW 39 0 0 Left 0
SYMATTR InstName V1
SYMATTR Value SINE(14.2 2 2)
SYMBOL res 224 -96 R90
WINDOW 0 0 56 VBottom 0
WINDOW 3 32 56 VTop 0
SYMATTR InstName R1
SYMATTR Value 220k
SYMBOL res 48 48 R90
WINDOW 0 0 56 VBottom 0
WINDOW 3 32 56 VTop 0
SYMATTR InstName R2
SYMATTR Value 1k
SYMBOL pmos 384 -208 M270
WINDOW 0 88 67 VLeft 0
WINDOW 3 120 69 VLeft 0
SYMATTR InstName M1
SYMATTR Value FDS4435A
SYMBOL res 432 -176 R0
SYMATTR InstName R3
SYMATTR Value 20k
SYMBOL res 432 64 R0
SYMATTR InstName R4
SYMATTR Value 10k
SYMBOL res 592 -32 R0
SYMATTR InstName R6
SYMATTR Value 6
SYMBOL zener -96 224 R180
WINDOW 0 -24 74 Left 0
WINDOW 3 -72 0 Left 0
SYMATTR InstName D1
SYMATTR Value 1N750
SYMATTR Description Diode
SYMATTR Type diode
SYMBOL pnp -176 -32 M180
SYMATTR InstName Q1
SYMATTR Value 2N3906
SYMBOL zener -192 -160 R180
WINDOW 0 24 72 Left 0
WINDOW 3 24 0 Left 0
SYMATTR InstName D2
SYMATTR Value 1N750
SYMATTR Description Diode
SYMATTR Type diode
SYMBOL res -128 -240 R0
SYMATTR InstName R7
SYMATTR Value 390
SYMBOL res -224 -64 R0
SYMATTR InstName R8
SYMATTR Value 1k
SYMBOL res 384 272 R90
WINDOW 0 0 56 VBottom 0
WINDOW 3 32 56 VTop 0
SYMATTR InstName R9
SYMATTR Value 0.01
SYMBOL cap 512 0 R0
SYMATTR InstName C1
SYMATTR Value 100µ
SYMBOL cap -224 144 R0
SYMATTR InstName C2
SYMATTR Value 1µ
SYMBOL Opamps\\opamp2 160 16 R0
SYMATTR InstName U1
SYMATTR Value lm358
SYMBOL res 288 -144 R0
SYMATTR InstName R10
SYMATTR Value 1k
TEXT -162 328 Left 0 !.tran 1
TEXT 776 -856 Left 0 !
*//////////////////////////////////////////////////////////\n*LM358
DUAL OPERATIONAL AMPLIFIER MACRO-MODEL
\n*//////////////////////////////////////////////////////////\n*\n*
connections: non-inverting input\n* |
inverting input\n* | | positive power supply
\n* | | | negative power supply
\n* | | | | output\n* |
| | | |\n* | | | | |\n.SUBCKT
LM358 1 2 99 50 28\n*\n*Features:\n*Eliminates need for
dual supplies\n*Large DC voltage gain = 100dB\n*High
bandwidth = 1MHz\n*Low input offset voltage
= 2mV\n*Wide supply range = +-1.5V to +-16V\n*
\n*NOTE: Model is for single device only and simulated\n* supply
current is 1/2 of total device current.\n* Output crossover
distortion with dual supplies\n* is not modeled.\n*
\n****************INPUT STAGE**************\n*\nIOS 2 1 5N\n*^Input
offset current\nR1 1 3 500K\nR2 3 2 500K\nI1 99 4 100U\nR3 5 50
517\nR4 6 50 517\nQ1 5 2 4 QX\nQ2 6 7 4 QX\n*Fp2=1.2 MHz\nC4 5 6
128.27P\n*\n***********COMMON MODE EFFECT***********\n*\nI2 99 50 75U
\n*^Quiescent supply current\nEOS 7 1 POLY(1) 16 49 2E-3 1\n*Input
offset voltage.^\nR8 99 49 60K\nR9 49 50 60K\n*\n*********OUTPUT
VOLTAGE LIMITING********\nV2 99 8 1.63\nD1 9 8 DX\nD2 10 9 DX\nV3 10
50 .635\n*\n**************SECOND STAGE**************\n*\nEH 99 98 99
49 1\nG1 98 9 POLY(1) 5 6 0 9.8772E-4 0 .3459\n*Fp1=7.86 Hz\nR5 98 9
101.2433MEG\nC3 98 9 200P\n*\n***************POLE STAGE***************
\n*\n*Fp=2 MHz\nG3 98 15 9 49 1E-6\nR12 98 15 1MEG\nC5 98 15
7.9577E-14\n*\n*********COMMON-MODE ZERO STAGE*********\n*\n*Fpcm=10
KHz\nG4 98 16 3 49 5.6234E-8 \nL2 98 17 15.9M\nR13 17 16
1K\n*\n**************OUTPUT STAGE**************\n*\nF6 50 99 POLY(1)
V6 300U 1\nE1 99 23 99 15 1\nR16 24 23 17.5\nD5 26 24 DX\nV6 26 22 .63V
\nR17 23 25 17.5\nD6 25 27 DX\nV7 22 27 .63V\nV5 22 21 0.27V\nD4 21 15
DX\nV4 20 22 0.27V\nD3 15 20 DX\nL3 22 28 500P\nRL3 22 28 100K\n*
\n***************MODELS USED**************\n*\n.MODEL DX
D(IS=1E-15)\n.MODEL QX PNP(BF=1.111E3)\n*\n.ENDS
From: Tim Wescott on 3 Jun 2010 12:12
On 06/03/2010 07:02 AM, neddie wrote:
> Hi to all.
> I've put together( well simulated :0) ) a discrete LDO cct using an
> opamp(lm358) and a p-channel mosfet.
> Under certain conditions it oscillates. How do I go about compensating
> this circuit so that it does not oscillate.
> I must stress I'm not going to actually build this circuit , I'm just
> playing around.
> If R9 (0.01) ohms is increaced to about 0.1 ohms or higher the circuit
> oscillates.Is it even possible to
> fix this , or is the circuit inherently unstable.As I aid I'm not
> going to build it ,I can buy a LDO :0) , I'm just playing around.The
> left part of the cct is just a constant current source feeding a zener
> for the voltage reference.
> The cct is in LTSpice format. LM358 model included.Just copy and put
> into -> file.asc

File's mangled, or at least my LTSpice couldn't read it.

Can you post an image somewhere, or a PDF?

Are you just feeding back voltage from the MOSFET drain, or are you
feeding back from its gate? It can be really difficult to get stability
with a common-source MOSFET circuit unless you have some sort of
feedback around the device, as well as around the whole circuit.

--
Tim Wescott
Control system and signal processing consulting
www.wescottdesign.com
From: neddie on 4 Jun 2010 02:42
On Jun 3, 6:12 pm, Tim Wescott <t...(a)seemywebsite.now> wrote:
> On 06/03/2010 07:02 AM, neddie wrote:
>
> > Hi to all.
> > I've put together( well simulated :0) )  a discrete LDO cct using an
> > opamp(lm358) and a p-channel mosfet.
> > Under certain conditions it oscillates. How do I go about compensating
> > this circuit so that it does not oscillate.
> > I must stress I'm not going to actually build this circuit , I'm just
> > playing around.
> > If R9 (0.01) ohms is increaced to about 0.1 ohms or higher the circuit
> > oscillates.Is it even possible to
> > fix this , or is the circuit inherently unstable.As I aid I'm not
> > going to build it ,I can buy a LDO :0) , I'm just playing around.The
> > left part of the cct is just a constant current source feeding a zener
> > for the voltage reference.
> > The cct is in LTSpice format. LM358 model included.Just copy and put
> > into ->   file.asc
>
> File's mangled, or at least my LTSpice couldn't read it.
>
> Can you post an image somewhere, or a PDF?
>
> Are you just feeding back voltage from the MOSFET drain, or are you
> feeding back from its gate?  It can be really difficult to get stability
> with a common-source MOSFET circuit unless you have some sort of
> feedback around the device, as well as around the whole circuit.
>
> --
> Tim Wescott
> Control system and signal processing consultingwww.wescottdesign.com

I'll try and post the 2 parts of the cct seperately.The cct and the
lm358 model.
I've made some progress. Adding a small cap (1n) across the feedback
resistor to reduce the higher
frequency gain has made this cct stable.I'd like to add a NPN
transistor across R9 to act as a current limit
cct. Emitter to gnd , base to other side of R9 and the collector to
the net marked Vcl. The idea is that when the volt drop across R9
is large enough to turn on the transistor , it pulls the output
voltage down.
This is the origonal cct.(without any add-ons).
This can be stabalised by adding a 1n cap across R11 , feedback
resistor. If I add the current limiting cct however , I can't
stabalise it.
Version 4
SHEET 1 3536 2432
WIRE -112 -256 -208 -256
WIRE -64 -256 -112 -256
WIRE 288 -256 -64 -256
WIRE 448 -256 384 -256
WIRE 528 -256 448 -256
WIRE 608 -256 528 -256
WIRE -208 -224 -208 -256
WIRE -112 -224 -112 -256
WIRE 448 -160 448 -256
WIRE -112 -128 -112 -144
WIRE 304 -128 304 -208
WIRE -208 -80 -208 -160
WIRE -176 -80 -208 -80
WIRE 176 -64 80 -64
WIRE -208 -48 -208 -80
WIRE 608 -48 608 -256
WIRE 608 -16 608 -48
WIRE -464 0 -464 -48
WIRE 528 0 528 -256
WIRE 448 16 448 -80
WIRE 448 16 352 16
WIRE -208 48 -208 32
WIRE -112 64 -112 -32
WIRE -48 64 -112 64
WIRE 80 64 80 -64
WIRE 80 64 32 64
WIRE 128 64 80 64
WIRE 256 80 256 -64
WIRE 256 80 192 80
WIRE 304 80 304 -48
WIRE 304 80 256 80
WIRE 448 80 448 16
WIRE -464 96 -464 80
WIRE 128 96 32 96
WIRE -112 112 -112 64
WIRE -112 112 -208 112
WIRE -208 144 -208 112
WIRE -112 160 -112 112
WIRE 32 176 32 96
WIRE 352 176 352 16
WIRE 352 176 32 176
WIRE -208 288 -208 208
WIRE -112 288 -112 224
WIRE -112 288 -208 288
WIRE 160 288 160 112
WIRE 160 288 -112 288
WIRE 288 288 160 288
WIRE 448 288 448 160
WIRE 448 288 368 288
WIRE 528 288 528 64
WIRE 528 288 448 288
WIRE 608 288 608 64
WIRE 608 288 528 288
WIRE 160 304 160 288
FLAG 160 304 0
FLAG -464 96 0
FLAG -464 -48 SUPPLY
FLAG -208 48 0
FLAG 608 -48 Load
FLAG -64 -256 SUPPLY
FLAG 80 64 Vcl
FLAG 160 48 SUPPLY
SYMBOL voltage -464 -16 R0
WINDOW 123 0 0 Left 0
WINDOW 39 0 0 Left 0
SYMATTR InstName V1
SYMATTR Value SINE(14.2 2 2)
SYMBOL res 48 48 R90
WINDOW 0 0 56 VBottom 0
WINDOW 3 32 56 VTop 0
SYMATTR InstName R2
SYMATTR Value 1k
SYMBOL pmos 384 -208 M270
WINDOW 0 88 67 VLeft 0
WINDOW 3 120 69 VLeft 0
SYMATTR InstName M1
SYMATTR Value FDS4435A
SYMBOL res 432 -176 R0
SYMATTR InstName R3
SYMATTR Value 20k
SYMBOL res 432 64 R0
SYMATTR InstName R4
SYMATTR Value 10k
SYMBOL res 592 -32 R0
SYMATTR InstName R6
SYMATTR Value 3
SYMBOL zener -96 224 R180
WINDOW 0 -24 74 Left 0
WINDOW 3 -72 0 Left 0
SYMATTR InstName D1
SYMATTR Value 1N750
SYMATTR Description Diode
SYMATTR Type diode
SYMBOL pnp -176 -32 M180
SYMATTR InstName Q1
SYMATTR Value 2N3906
SYMBOL zener -192 -160 R180
WINDOW 0 24 72 Left 0
WINDOW 3 24 0 Left 0
SYMATTR InstName D2
SYMATTR Value 1N750
SYMATTR Description Diode
SYMATTR Type diode
SYMBOL res -128 -240 R0
SYMATTR InstName R7
SYMATTR Value 390
SYMBOL res -224 -64 R0
SYMATTR InstName R8
SYMATTR Value 1k
SYMBOL res 384 272 R90
WINDOW 0 0 56 VBottom 0
WINDOW 3 32 56 VTop 0
SYMATTR InstName R9
SYMATTR Value 0.1
SYMBOL cap 512 0 R0
SYMATTR InstName C1
SYMATTR Value 1000µ
SYMBOL cap -224 144 R0
SYMATTR InstName C2
SYMATTR Value 1µ
SYMBOL Opamps\\opamp2 160 16 R0
SYMATTR InstName U1
SYMATTR Value lm358
SYMBOL res 288 -144 R0
SYMATTR InstName R10
SYMATTR Value 1k
SYMBOL res 272 -80 R90
WINDOW 0 0 56 VBottom 0
WINDOW 3 32 56 VTop 0
SYMATTR InstName R11
SYMATTR Value 220k
TEXT -162 328 Left 0 !.tran 1
TEXT -504 192 Left 0 !;ac dec 1000 1 10meg


This is the LM358 model.
*//////////////////////////////////////////////////////////
*LM358 DUAL OPERATIONAL AMPLIFIER MACRO-MODEL
*//////////////////////////////////////////////////////////
*
* connections: non-inverting input
* | inverting input
* | | positive power supply
* | | | negative power supply
* | | | | output
* | | | | |
* | | | | |
..SUBCKT LM358 1 2 99 50 28
*
*Features:
*Eliminates need for dual supplies
*Large DC voltage gain = 100dB
*High bandwidth = 1MHz
*Low input offset voltage = 2mV
*Wide supply range = +-1.5V to +-16V
*
*NOTE: Model is for single device only and simulated
* supply current is 1/2 of total device current.
* Output crossover distortion with dual supplies
* is not modeled.
*
****************INPUT STAGE**************
*
IOS 2 1 5N
*^Input offset current
R1 1 3 500K
R2 3 2 500K
I1 99 4 100U
R3 5 50 517
R4 6 50 517
Q1 5 2 4 QX
Q2 6 7 4 QX
*Fp2=1.2 MHz
C4 5 6 128.27P
*
***********COMMON MODE EFFECT***********
*
I2 99 50 75U
*^Quiescent supply current
EOS 7 1 POLY(1) 16 49 2E-3 1
*Input offset voltage.^
R8 99 49 60K
R9 49 50 60K
*
*********OUTPUT VOLTAGE LIMITING********
V2 99 8 1.63
D1 9 8 DX
D2 10 9 DX
V3 10 50 .635
*
**************SECOND STAGE**************
*
EH 99 98 99 49 1
G1 98 9 POLY(1) 5 6 0 9.8772E-4 0 .3459
*Fp1=7.86 Hz
R5 98 9 101.2433MEG
C3 98 9 200P
*
***************POLE STAGE***************
*
*Fp=2 MHz
G3 98 15 9 49 1E-6
R12 98 15 1MEG
C5 98 15 7.9577E-14
*
*********COMMON-MODE ZERO STAGE*********
*
*Fpcm=10 KHz
G4 98 16 3 49 5.6234E-8
L2 98 17 15.9M
R13 17 16 1K
*
**************OUTPUT STAGE**************
*
F6 50 99 POLY(1) V6 300U 1
E1 99 23 99 15 1
R16 24 23 17.5
D5 26 24 DX
V6 26 22 .63V
R17 23 25 17.5
D6 25 27 DX
V7 22 27 .63V
V5 22 21 0.27V
D4 21 15 DX
V4 20 22 0.27V
D3 15 20 DX
L3 22 28 500P
RL3 22 28 100K
*
***************MODELS USED**************
*
..MODEL DX D(IS=1E-15)
..MODEL QX PNP(BF=1.111E3)
*
..ENDS
 | 
Pages: 1
Prev: When and Why use resistors between base and emitter
Next: Phase shift in cabling?