On Fri, 23 Mar 2007 18:13:20 -0500, John Fields
wrote:
On Sat, 24 Mar 2007 07:48:15 +1100, Trevor Morton
wrote:
I am sorry if my original post caused confusion. Yes, I am looking for
an op amp because of the DC requirement. The LM386 I tried to no
avail.
Phrasing it another way, why is there no predefined op amp/Darlington
configuration, or off-the-shelf power op amp, that can do the all same
things as the LM386 does with audio, but down to DC?
Single supply
Wide supplyvoltage range
DC coupling
Easily variable gain
Low power
Etc.
At first, I thought such specs might be mutually exclusive. However,
if there are such design limitations, they seem to be receeding fast.
For example, there is the "new" OPA569, rated at 2A. It meets all my
requirements except it only operates up to 5.5V. I need a range of 4
to 12V.
http://focus.ti.com/docs/prod/folders/print/opa569.html
Can someone explain, why can't something like this be done with a
CA3130 and a couple of transistors.
---
Please bottom post.
The main reason you can't get what you want is because you're
specifying a single supply and DC coupling and expecting an output
which can swing negative.
For example, this circuit:
Version 4
SHEET 1 880 680
WIRE -192 64 -416 64
WIRE -80 64 -112 64
WIRE 64 64 -80 64
WIRE 336 64 144 64
WIRE -80 192 -80 64
WIRE -48 192 -80 192
WIRE 64 208 16 208
WIRE 176 208 144 208
WIRE -240 224 -272 224
WIRE -192 224 -240 224
WIRE -80 224 -112 224
WIRE -48 224 -80 224
WIRE 240 272 240 256
WIRE 336 272 336 64
WIRE 336 272 240 272
WIRE -416 288 -416 64
WIRE -240 288 -240 224
WIRE -80 288 -80 224
WIRE 240 304 240 272
WIRE -416 416 -416 368
WIRE -240 416 -240 368
WIRE -240 416 -416 416
WIRE -80 416 -80 368
WIRE -80 416 -240 416
WIRE -16 416 -16 240
WIRE -16 416 -80 416
WIRE 240 416 240 384
WIRE 240 416 -16 416
WIRE -416 480 -416 416
FLAG -416 480 0
FLAG -272 224 +12V
FLAG 240 160 +12V
FLAG -16 176 +12V
SYMBOL Opamps\\LT1055 -16 144 R0
WINDOW 0 30 88 Left 0
WINDOW 3 27 117 Left 0
SYMATTR InstName U1
SYMBOL npn 176 160 R0
SYMATTR InstName Q1
SYMATTR Value 2N3904
SYMBOL res 224 288 R0
SYMATTR InstName R2
SYMATTR Value 20
SYMBOL res -96 272 R0
SYMATTR InstName R3
SYMATTR Value 10K
SYMBOL res -96 208 R90
WINDOW 0 94 56 VBottom 0
WINDOW 3 32 56 VTop 0
SYMATTR InstName R4
SYMATTR Value 10K
SYMBOL voltage -240 272 R0
WINDOW 0 -50 100 Left 0
WINDOW 3 24 104 Invisible 0
WINDOW 123 0 0 Left 0
WINDOW 39 0 0 Left 0
SYMATTR InstName V1
SYMATTR Value 12
SYMBOL voltage -416 272 R0
WINDOW 0 -50 100 Left 0
WINDOW 3 24 104 Invisible 0
WINDOW 123 0 0 Left 0
WINDOW 39 0 0 Left 0
SYMATTR InstName V2
SYMATTR Value SINE(6 .1 10)
SYMBOL res -96 48 R90
WINDOW 0 -36 49 VBottom 0
WINDOW 3 -35 52 VTop 0
SYMATTR InstName R5
SYMATTR Value 1K
SYMBOL res 160 192 R90
WINDOW 0 -41 58 VBottom 0
WINDOW 3 -37 56 VTop 0
SYMATTR InstName R6
SYMATTR Value 1K
SYMBOL res 160 48 R90
WINDOW 0 -39 56 VBottom 0
WINDOW 3 -34 57 VTop 0
SYMATTR InstName R1
SYMATTR Value 10K
TEXT -396 450 Left 0 !.tran 1 uic
will do what you want, but because of the single supply and no
capacitor rule, the input signal must be offset and the output
signal will swing about a DC value, which will waste power in the
load as well as in the single (which is what you asked for) output
transistor.
The circuit list, above, is for LTSPICE, and if you don't have a
copy you can download one, free, at:
http://www.linear.com/designtools/software/
One caveat: the output transistor I used can't dissipate the power
needed to feed the 20 ohm load shown, but I didn't want to spend a
lot of time looking for the right one this early in the game.
---
This is better:
Version 4
SHEET 1 880 680
WIRE -16 -32 -336 -32
WIRE 240 -32 -16 -32
WIRE -448 0 -480 0
WIRE -272 0 -368 0
WIRE -336 32 -336 -32
WIRE -608 48 -640 48
WIRE -480 48 -480 0
WIRE -480 48 -528 48
WIRE -368 48 -480 48
WIRE -272 64 -272 0
WIRE -272 64 -304 64
WIRE -208 64 -272 64
WIRE -80 64 -128 64
WIRE 64 64 -80 64
WIRE 272 64 144 64
WIRE -368 80 -480 80
WIRE -240 160 -432 160
WIRE -16 160 -16 -32
WIRE -16 160 -240 160
WIRE 240 160 240 -32
WIRE -432 192 -432 160
WIRE -16 208 -16 160
WIRE -80 224 -80 64
WIRE -48 224 -80 224
WIRE 64 240 16 240
WIRE 192 240 144 240
WIRE -240 256 -240 160
WIRE -192 256 -240 256
WIRE -80 256 -112 256
WIRE -48 256 -80 256
WIRE 240 272 240 256
WIRE 272 272 272 64
WIRE 272 272 240 272
WIRE -480 304 -480 80
WIRE -432 304 -432 272
WIRE -432 304 -480 304
WIRE -640 320 -640 48
WIRE -432 320 -432 304
WIRE -240 320 -240 256
WIRE -80 320 -80 256
WIRE 240 320 240 272
WIRE -640 448 -640 400
WIRE -432 448 -432 400
WIRE -432 448 -640 448
WIRE -336 448 -336 96
WIRE -336 448 -432 448
WIRE -240 448 -240 400
WIRE -240 448 -336 448
WIRE -80 448 -80 400
WIRE -80 448 -240 448
WIRE -16 448 -16 272
WIRE -16 448 -80 448
WIRE 240 448 240 400
WIRE 240 448 -16 448
WIRE -640 512 -640 448
FLAG -640 512 0
SYMBOL res -96 304 R0
WINDOW 0 -40 46 Left 0
WINDOW 3 -47 77 Left 0
SYMATTR InstName R3
SYMATTR Value 10K
SYMBOL res -96 240 R90
WINDOW 0 -33 58 VBottom 0
WINDOW 3 -33 59 VTop 0
SYMATTR InstName R4
SYMATTR Value 10k
SYMBOL voltage -240 304 R0
WINDOW 0 -50 100 Left 0
WINDOW 3 24 104 Invisible 0
WINDOW 123 0 0 Left 0
WINDOW 39 0 0 Left 0
SYMATTR InstName V1
SYMATTR Value 12
SYMBOL voltage -640 304 R0
WINDOW 0 -50 100 Left 0
WINDOW 3 24 104 Invisible 0
WINDOW 123 0 0 Left 0
WINDOW 39 0 0 Left 0
SYMATTR InstName V2
SYMATTR Value SINE(0 .4 10)
SYMBOL res -112 48 R90
WINDOW 0 -36 49 VBottom 0
WINDOW 3 -35 52 VTop 0
SYMATTR InstName R5
SYMATTR Value 1K
SYMBOL res 160 224 R90
WINDOW 0 -27 57 VBottom 0
WINDOW 3 -26 56 VTop 0
SYMATTR InstName R6
SYMATTR Value 1K
SYMBOL res 160 48 R90
WINDOW 0 -39 56 VBottom 0
WINDOW 3 -34 57 VTop 0
SYMATTR InstName R1
SYMATTR Value 10K
SYMBOL res -352 -16 R90
WINDOW 0 -36 54 VBottom 0
WINDOW 3 -29 58 VTop 0
SYMATTR InstName R7
SYMATTR Value 10k
SYMBOL res -512 32 R90
WINDOW 0 -37 54 VBottom 0
WINDOW 3 -34 56 VTop 0
SYMATTR InstName R8
SYMATTR Value 10k
SYMBOL res -416 288 R180
WINDOW 0 -59 74 Left 0
WINDOW 3 -51 44 Left 0
SYMATTR InstName R10
SYMATTR Value 9k
SYMBOL res -448 304 R0
SYMATTR InstName R9
SYMATTR Value 3k
SYMBOL nmos 192 160 R0
WINDOW 0 -34 47 Left 0
WINDOW 3 -76 -9 Left 0
SYMATTR InstName M1
SYMATTR Value IRF6635
SYMBOL Opamps\\LT1218 -336 0 R0
WINDOW 0 -76 100 Left 0
WINDOW 3 -100 126 Left 0
SYMATTR InstName U3
SYMBOL Opamps\\LT1218 -16 176 R0
WINDOW 0 32 86 Left 0
WINDOW 3 9 115 Left 0
SYMATTR InstName U1
SYMBOL res 224 304 R0
WINDOW 0 -40 46 Left 0
WINDOW 3 -36 74 Left 0
SYMATTR InstName R2
SYMATTR Value 20
TEXT -624 480 Left 0 !.tran 1 uic
The source is DC coupled and ground-referred, and since you're using
a single positive DC supply, the output is operating
emitter-follower class A and varies about a quiescent Vcc/2.
--
JF