Maker Pro
Maker Pro

Op amps CSS check pls.

R

Robert Quinn

Jan 1, 1970
0
The circuit linked below is intended to be a CCS amp with a gain of 2
and double pole LPF. The latter smooths an eight step, digitally
simulated sinewave (8 Hz, 1.5V swing at pin 3). Load is a 200 Ohm air
core solenoid.

The finished device will be a field calibration source for my Earth
micropulsation sensing coils. It operates from 4 x AA cells.

This is as far as I have progessed, based upon a combination of
existing circuits. I am having trouble finding the relevant
information to proceed.

http://home.iprimus.com.au/loungecinema/css.gif

1. It only outputs unity gain. With the LM358 I am hoping for at least
5V.

2. The 1.5V sinewave looks fine, until a load is connected. At 200R
the upper right half the the waveform distorts. At 25R the output goes
flat. As monitored across the load.

Any advice would be sincerely appreciated.

Robert Quinn
 
J

John O'Flaherty

Jan 1, 1970
0
The circuit linked below is intended to be a CCS amp with a gain of 2
and double pole LPF. The latter smooths an eight step, digitally
simulated sinewave (8 Hz, 1.5V swing at pin 3). Load is a 200 Ohm air
core solenoid.

The finished device will be a field calibration source for my Earth
micropulsation sensing coils. It operates from 4 x AA cells.

This is as far as I have progessed, based upon a combination of
existing circuits. I am having trouble finding the relevant
information to proceed.

http://home.iprimus.com.au/loungecinema/css.gif

1. It only outputs unity gain. With the LM358 I am hoping for at least
5V.

2. The 1.5V sinewave looks fine, until a load is connected. At 200R
the upper right half the the waveform distorts. At 25R the output goes
flat. As monitored across the load.

The FET is shown with source and drain interchanged. Even connected
correctly, the 2N2700 has a typical gate cutoff voltage of 2.1 V,
which means it needs to stay that far above the transistor base, and
2.8 V higher than the output on the coil. You would do better with a
higher power supply voltage, like 9 or 12 V. (Unless you've selected a
transistor with especially low cutoff voltage- the range is pretty
wide.) That it works at all with a 200 ohm load probably means you've
got a FET with low cutoff voltage.

The amplifier, besides being a filter, is also a unity-gain voltage
follower below cutoff. It's a constant voltage source for the output
on the top of the load coil, so there's no gain of 2. You would need a
different circuit configuration to get a constant current source.

The filter has a cutoff frequency of about 4 Hz, a little low for an 8
Hz input signal.

With a load of only 25 ohms and 3 V p-p output, you might be pushing
the 2N2222 a little hard without a heat sink.

If I can suggest something, you might split up the functions- filter,
gain amplifier, constant current source- into separate stages until
everything is working, and then condense it if possible. It's easier
to troubleshoot things piecemeal.

I simulated the circuit in LTspice, which you can download for free
(just Google it). The .asc file, which I'll send in another post, can
be loaded and will display the schematic, and you can simulate it. You
can get an undistorted output with either a 200 ohm or 25 ohm load by
changing the power supply voltage from 6 to 12V, or by changing the
FET to an FDS6574A, which has a low (0.65 V) cutoff voltage. You can
change this by right-clicking the FET symbol.
 
J

John O'Flaherty

Jan 1, 1970
0
The simulator file- copy the text and save it into notepad, and save
it with an extension of .asc, and then load it into LTspice.

Version 4
SHEET 1 880 680
WIRE -1312 -1008 -1504 -1008
WIRE -1168 -1008 -1312 -1008
WIRE -800 -1008 -1056 -1008
WIRE -688 -1008 -800 -1008
WIRE -1312 -992 -1312 -1008
WIRE -800 -992 -800 -1008
WIRE -1504 -976 -1504 -1008
WIRE -1056 -976 -1056 -1008
WIRE -1056 -976 -1232 -976
WIRE -1056 -960 -1056 -976
WIRE -912 -944 -992 -944
WIRE -864 -944 -912 -944
WIRE -688 -928 -688 -1008
WIRE -1232 -912 -1232 -976
WIRE -912 -912 -912 -944
WIRE -1312 -896 -1312 -912
WIRE -1264 -896 -1312 -896
WIRE -1168 -880 -1168 -1008
WIRE -1168 -880 -1200 -880
WIRE -1104 -880 -1168 -880
WIRE -1600 -864 -1632 -864
WIRE -1504 -864 -1504 -912
WIRE -1504 -864 -1520 -864
WIRE -1488 -864 -1504 -864
WIRE -1376 -864 -1408 -864
WIRE -1264 -864 -1376 -864
WIRE -992 -864 -992 -944
WIRE -992 -864 -1056 -864
WIRE -1632 -848 -1632 -864
WIRE -1376 -816 -1376 -864
WIRE -1312 -816 -1312 -896
WIRE -912 -816 -912 -832
WIRE -912 -816 -1312 -816
WIRE -800 -816 -800 -896
WIRE -800 -816 -912 -816
WIRE -688 -816 -688 -848
WIRE -912 -800 -912 -816
WIRE -1232 -768 -1232 -848
WIRE -1184 -768 -1232 -768
WIRE -1184 -736 -1184 -768
WIRE -1632 -704 -1632 -768
WIRE -1376 -704 -1376 -752
WIRE -1376 -704 -1632 -704
WIRE -1232 -704 -1232 -768
WIRE -1232 -704 -1376 -704
WIRE -912 -704 -912 -720
WIRE -912 -704 -1232 -704
FLAG -688 -816 0
FLAG -1184 -736 0
SYMBOL npn -864 -992 R0
SYMATTR InstName Q1
SYMATTR Value 2N2222
SYMBOL res -1504 -880 R90
WINDOW 0 0 56 VBottom 0
WINDOW 3 32 56 VTop 0
SYMATTR InstName R1
SYMATTR Value 33k
SYMBOL res -1392 -880 R90
WINDOW 0 0 56 VBottom 0
WINDOW 3 32 56 VTop 0
SYMATTR InstName R2
SYMATTR Value 33k
SYMBOL cap -1520 -976 R0
WINDOW 3 38 44 Left 0
SYMATTR Value 1µ
SYMATTR InstName C1
SYMBOL res -1296 -896 R180
WINDOW 0 36 76 Left 0
WINDOW 3 36 40 Left 0
SYMATTR InstName R3
SYMATTR Value 22k
SYMBOL Opamps\\UniversalOpamp -1232 -880 R0
SYMATTR InstName U1
SYMBOL cap -1392 -816 R0
SYMATTR InstName C2
SYMATTR Value 1µ
SYMBOL res -928 -928 R0
WINDOW 3 38 56 Left 0
SYMATTR Value 10k
SYMATTR InstName R4
SYMBOL res -928 -816 R0
SYMATTR InstName R5
SYMATTR Value 200
SYMBOL voltage -688 -944 R0
WINDOW 123 0 0 Left 0
WINDOW 39 0 0 Left 0
SYMATTR InstName V1
SYMATTR Value 6
SYMBOL voltage -1632 -864 R0
WINDOW 0 -43 6 Left 0
WINDOW 3 -59 105 Left 0
WINDOW 123 0 0 Left 0
WINDOW 39 0 0 Left 0
SYMATTR InstName V2
SYMATTR Value SINE(3 1.5 8)
SYMBOL nmos -1104 -960 R0
SYMATTR InstName M2
SYMATTR Value IRF3706
TEXT -1664 -1080 Left 0 !.lib opamp.sub
TEXT -1640 -992 Left 0 !.tran 10
 
Top