Negative Cap Clapp oscillator

B

bitrex

Guest
If you massage this old bus-terminator speeder-upper negative
capacitance idea:

<http://analoglib.net/2013-05-30/shoji-jssc-85-negative-capacitance-bus-terminator/>

into a Clapp oscillator topology you get the Negative Cap Clapp which
sounds like a very bad type of venereal disease to get.

It makes even low-Q coils ring just set the R looking into the negative
C about equal to the ESR, make the positive parallel capacitance
slightly larger than the equivalent negative and watch it rip.

It tends to be finicky with component choices though and likes to head
for the rails or bounce around at low frequency if you pick "bad" ones
due to the lack of a DC feedback path between points A, and B at the
lower op-amp output; it's not obvious to me how to improve that and
suggestions are appreciated

Version 4
SHEET 1 1940 2228
WIRE 1488 -320 1488 -368
WIRE 608 -304 608 -368
WIRE 608 -304 -176 -304
WIRE 992 -304 608 -304
WIRE 1456 -304 992 -304
WIRE 1712 -288 1520 -288
WIRE 1824 -288 1712 -288
WIRE 1456 -272 1328 -272
WIRE 1488 -208 1488 -256
WIRE 992 -144 992 -304
WIRE -176 -80 -176 -304
WIRE -704 -64 -704 -96
WIRE -496 -64 -496 -96
WIRE 608 0 608 -304
WIRE -704 64 -704 16
WIRE -496 64 -496 16
WIRE 992 112 992 -64
WIRE -176 176 -176 0
WIRE 608 320 608 64
WIRE 976 320 608 320
WIRE 1328 320 1328 -272
WIRE 1328 320 1056 320
WIRE 1712 320 1712 -288
WIRE 1712 320 1328 320
WIRE -176 432 -176 256
WIRE -176 432 -464 432
WIRE 48 432 -176 432
WIRE 304 432 112 432
WIRE 64 688 64 672
WIRE 608 688 608 320
WIRE -464 704 -464 432
WIRE -176 704 -176 432
WIRE 32 704 -176 704
WIRE 304 720 304 432
WIRE 304 720 96 720
WIRE 32 736 -176 736
WIRE 64 784 64 752
WIRE -176 960 -176 736
WIRE 32 960 -176 960
WIRE 304 960 304 720
WIRE 304 960 112 960
WIRE 416 960 304 960
WIRE -176 1104 -176 960
WIRE -464 1312 -464 768
WIRE -176 1312 -176 1184
WIRE 608 1328 608 752
FLAG -496 -96 VCC
FLAG -496 64 0
FLAG -704 64 0
FLAG -704 -96 VEE
FLAG 608 1328 0
FLAG 1488 -208 VCC
FLAG 64 784 VCC
FLAG 64 672 VEE
FLAG 992 112 0
FLAG -176 1312 0
FLAG 1488 -368 VEE
FLAG -464 1312 0
FLAG 608 -368 A
FLAG 416 960 B
FLAG 1824 -288 OUT
IOPIN 1824 -288 Out
SYMBOL voltage -496 -80 R0
WINDOW 123 0 0 Left 0
WINDOW 39 0 0 Left 0
SYMATTR InstName V2
SYMATTR Value 5
SYMBOL voltage -704 -80 R0
WINDOW 123 0 0 Left 0
WINDOW 39 0 0 Left 0
SYMATTR InstName V1
SYMATTR Value -5
SYMBOL res 128 944 R90
WINDOW 0 0 56 VBottom 2
WINDOW 3 32 56 VTop 2
SYMATTR InstName R7
SYMATTR Value 10k
SYMBOL ind -160 16 R180
WINDOW 0 36 80 Left 2
WINDOW 3 36 40 Left 2
SYMATTR InstName L2
SYMATTR Value 2Âľ
SYMATTR SpiceLine Rser=25
SYMBOL cap 592 0 R0
SYMATTR InstName C5
SYMATTR Value 47n
SYMBOL cap 592 688 R0
SYMATTR InstName C6
SYMATTR Value 47n
SYMBOL res 1072 304 R90
WINDOW 0 0 56 VBottom 2
WINDOW 3 32 56 VTop 2
SYMATTR InstName R4
SYMATTR Value 1k
SYMBOL res 976 -160 R0
SYMATTR InstName R5
SYMATTR Value 100k
SYMBOL res -192 1088 R0
SYMATTR InstName R8
SYMATTR Value 10k
SYMBOL cap 112 416 R90
WINDOW 0 0 32 VBottom 2
WINDOW 3 32 32 VTop 2
SYMATTR InstName C1
SYMATTR Value 10n
SYMBOL Opamps\\AD8034 64 784 M180
SYMATTR InstName U7
SYMBOL Opamps\\AD8034 1488 -224 M180
SYMATTR InstName U3
SYMBOL cap -480 704 R0
SYMATTR InstName C7
SYMATTR Value 11n
SYMBOL res -160 272 R180
WINDOW 0 36 76 Left 2
WINDOW 3 36 40 Left 2
SYMATTR InstName R9
SYMATTR Value 25
TEXT -704 160 Left 2 !.tran 100m startup uic
 
On 8/13/19 11:07 PM, bitrex wrote:
If you massage this old bus-terminator speeder-upper negative
capacitance idea:

http://analoglib.net/2013-05-30/shoji-jssc-85-negative-capacitance-bus-terminator/


into a Clapp oscillator topology you get the Negative Cap Clapp which
sounds like a very bad type of venereal disease to get.

It makes even low-Q coils ring just set the R looking into the negative
C about equal to the ESR, make the positive parallel capacitance
slightly larger than the equivalent negative and watch it rip.

It tends to be finicky with component choices though and likes to head
for the rails or bounce around at low frequency if you pick "bad" ones
due to the lack of a DC feedback path between points A, and B at the
lower op-amp output; it's not obvious to me how to improve that and
suggestions are appreciated
Click to expand...

Lowering R1 to 10k seems to help a lot
 
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