1
\$\begingroup\$

enter image description here

In the above image, there is a bistable multivibrator circuit in Figure 1. The output of stage 1 of this circuit at \$V_{o1} \$ is supposed to be a square wave with a peak to peak voltage of 15 V with zero average and a frequency of 10 kHz or 0.1 ms period.

The output is not working correctly. The input to this bistable multivibrator at \$V_-\$ terminal of Op amp \$U_2\$ from \$V_{c1}\$ is a supposed to be triangular wave with a peak to peak voltage of 7.5 V and frequency of 10 kHz or 0.1 ms period. \$V_{TH}\$ and \$V_{TL}\$ of the bistable multivibrator in Figure 1 are 3.75 V and -3.75 V at which the output of \$V_{o1}\$ is supposed to change state. The output at \$V_{o2}\$ is supposed to be a triangular wave with a peak to peak voltage of 15 V and 0.1 ms period. Op amp saturation voltages are at +13 V and -13 V.

I separated the circuit as Figure 2 and Figure 3 with their inputs and resistors as per the calculations. I don't get any output for Figure 2 at \$V_{o3}\$ as in Figure 1 at \$V_{o1}\$. The original circuit in Figure 1 has GND connected to resistor \$R_6\$. I tried giving a pulse of 1 ms period and 1 ms on time with a 1 V amplitude to make any oscillations happen, but it's not working.

What is the reason for this circuit to not function as expected? What modifications can you suggest to get square waves and triangular waves at \$V_{o1}\$ and \$V_{o2}\$ ?

Your responses are appreciated.

LTspice .asc file for the circuit is below:

Version 4
SHEET 1 2068 1620
WIRE 672 16 624 16
WIRE 880 16 752 16
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WINDOW 0 0 56 VBottom 2
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SYMATTR InstName R3
SYMATTR Value 100k
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SYMBOL res -496 256 R90
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WINDOW 0 24 64 Left 2
WINDOW 3 24 0 Left 2
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SYMATTR Type diode
SYMBOL zener -16 432 R0
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SYMATTR Value PTZ6_8B
SYMATTR Description Diode
SYMATTR Type diode
SYMBOL res 896 672 R90
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WINDOW 3 32 56 VTop 2
SYMATTR InstName R7
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WINDOW 39 0 0 Left 0
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SYMBOL OpAmps\\OP296 -256 352 M180
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SYMBOL OpAmps\\OP296 704 96 R0
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SYMBOL res -160 880 R90
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SYMATTR Value 5.3012k
SYMBOL zener -80 1008 R180
WINDOW 0 24 64 Left 2
WINDOW 3 24 0 Left 2
SYMATTR InstName D3
SYMATTR Value PTZ6_8B
SYMATTR Description Diode
SYMATTR Type diode
SYMBOL zener -112 1040 R0
SYMATTR InstName D4
SYMATTR Value PTZ6_8B
SYMATTR Description Diode
SYMATTR Type diode
SYMBOL OpAmps\\OP296 -352 960 M180
SYMATTR InstName U1
SYMBOL voltage -432 1120 R0
WINDOW 123 0 0 Left 0
WINDOW 39 0 0 Left 0
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SYMBOL voltage -944 896 R0
WINDOW 123 0 0 Left 0
WINDOW 39 0 0 Left 0
SYMATTR InstName V5
SYMATTR Value PULSE(0 1 0 0 0 1m 1m)
SYMBOL OpAmps\\OP296 832 768 R0
SYMATTR InstName U3
TEXT -788 1256 Left 2 !.tran 1m
TEXT -1088 1360 Left 2 ;Descrption: Bistable multivibrator circuit\nVersion: 0.1
TEXT 24 664 Left 2 ;Figure 1
TEXT -464 1384 Left 2 ;Figure 2
TEXT 584 1256 Left 2 ;Figure 3

Edit: After correcting the power supplies for first stage in Figure 1 and changing the OP amp, the following waveforms are obtained. For this application, an op amp with higher bandwidth is required. LT1056 seems to fit it exactly. Op296 op amp seemed to make the circuit in an indeterminate state. The only problem was that Voltage at \$V_{o1}\$ had about 8 V peak This is due to higher forward voltage drop of the zener diodes used. I had used a 6.8 V zener from ROHM with the model PTZ6_8B. The output signal had a frequency of about 9 KHz.This is mainly due to some errors in calculating \$V_{TH}\$ and \$V_{TL}\$ for the multivibrator exactly. Something can be done to change the frequency to near 10 KHz.

LTspice asc file for the circuit is below:

Version 4
SHEET 1 2068 1620
WIRE 672 16 624 16
WIRE 880 16 752 16
WIRE -1088 144 -1088 96
WIRE -288 144 -336 144
WIRE 0 144 -208 144
WIRE 368 144 112 144
WIRE 624 144 624 16
WIRE 624 144 448 144
WIRE 672 144 624 144
WIRE 1120 144 1104 144
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WINDOW 3 24 0 Left 2
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SYMATTR Type diode
SYMBOL zener -16 432 R0
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SYMATTR Description Diode
SYMATTR Type diode
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WINDOW 3 32 56 VTop 2
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WINDOW 3 24 0 Left 2
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SYMATTR Value PTZ6_8B
SYMATTR Description Diode
SYMATTR Type diode
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SYMATTR InstName D4
SYMATTR Value PTZ6_8B
SYMATTR Description Diode
SYMATTR Type diode
SYMBOL voltage -432 1120 R0
WINDOW 123 0 0 Left 0
WINDOW 39 0 0 Left 0
SYMATTR InstName V1
SYMATTR Value PULSE(-3.75 3.75 0 0.00005 0.00005 0 0.0001)
SYMBOL OpAmps\\LT1056 896 816 R0
SYMATTR InstName U2
SYMBOL OpAmps\\LT1056 704 96 R0
SYMATTR InstName U5
SYMBOL OpAmps\\LT1056 -256 352 M180
SYMATTR InstName U6
SYMBOL OpAmps\\LT1056 -352 960 M180
SYMATTR InstName U7
TEXT -788 1256 Left 2 !.tran 1m
TEXT -1088 1360 Left 2 ;Descrption: Bistable multivibrator circuit\nVersion: 0.1
TEXT 24 664 Left 2 ;Figure 1
TEXT -464 1384 Left 2 ;Figure 2
TEXT 584 1256 Left 2 ;Figure 3

Reference:

Microelectronic circuits, 7e, Sedra and Smith, OUP India, year 2017, page 1117

\$\endgroup\$
9
  • \$\begingroup\$ What happens if you remove R4? \$\endgroup\$
    – Jens
    Commented Oct 14 at 10:08
  • \$\begingroup\$ I removed \$R_4\$ equivalent \$R_{10} \$ in Figure 3, the gain of the second stage has increased. the output voltage at \$ V_{o4}\$ is about +8V to -5V approximately. There is no change in Figure 1 for removing \$R_4\$. There is only a DC voltage at \$ V_{o4}\$. \$\endgroup\$
    – Amit M
    Commented Oct 14 at 10:19
  • \$\begingroup\$ Removing R4 should enable the oscillation and removing R2 then should enable the square wave output of Vo2. \$\endgroup\$
    – Jens
    Commented Oct 14 at 10:24
  • \$\begingroup\$ If \$R_6\$is much lowe as 10 KΩ, then β of the multivibrator would change and that would also change change \$V_{TL}\$ and \$V_{TH}\$ , as a result, the rectangular wave might have different on time and off times. \$\endgroup\$
    – Amit M
    Commented Oct 14 at 10:28
  • 1
    \$\begingroup\$ You call it a bistable, but it looks like it's an astable? Is that just a terminology mistake or am I missing something here? \$\endgroup\$
    – Hearth
    Commented Oct 14 at 15:10

2 Answers 2

Reset to default
2
\$\begingroup\$

Here you have the working version: enter image description here

enter image description here

As you can see I change the opamp type LT1056. And correct a small mistake you made in connecting the supply voltage to U2 (U1 on my schematic).

LTspice .asc file for the circuit is below:

Version 4
SHEET 1 2068 1620
WIRE 608 16 528 16
WIRE 784 16 688 16
WIRE -32 144 -144 144
WIRE 192 144 48 144
WIRE 384 144 320 144
WIRE 528 144 528 16
WIRE 528 144 464 144
WIRE 576 144 528 144
WIRE 784 160 784 16
WIRE 784 160 640 160
WIRE 912 160 784 160
WIRE 576 176 528 176
WIRE 608 208 608 192
WIRE -64 256 -64 240
WIRE -288 272 -320 272
WIRE -144 272 -144 144
WIRE -144 272 -208 272
WIRE -96 272 -144 272
WIRE 48 288 -32 288
WIRE 192 288 192 144
WIRE 192 288 128 288
WIRE 272 288 192 288
WIRE 464 288 352 288
WIRE 496 288 464 288
WIRE 528 288 528 176
WIRE 528 288 496 288
WIRE 608 288 528 288
WIRE 784 288 784 160
WIRE 784 288 688 288
WIRE -96 304 -144 304
WIRE -496 320 -496 272
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WIRE 528 368 528 288
WIRE 192 432 192 400
WIRE -496 448 -496 400
WIRE -400 448 -496 448
WIRE 528 464 528 432
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WIRE -496 496 -496 448
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WIRE 464 608 -144 608
WIRE -496 624 -496 576
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FLAG 608 128 Vcc
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FLAG 528 464 0
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FLAG -320 272 0
FLAG 496 288 Vc1
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FLAG -64 240 Vee
FLAG -64 320 Vcc
SYMBOL voltage -496 304 R0
WINDOW 123 0 0 Left 0
WINDOW 39 0 0 Left 0
SYMATTR InstName V2
SYMATTR Value 13
SYMBOL voltage -496 480 R0
WINDOW 123 0 0 Left 0
WINDOW 39 0 0 Left 0
SYMATTR InstName V3
SYMATTR Value 13
SYMBOL res 704 0 R90
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WINDOW 3 32 56 VTop 2
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SYMATTR Value 100k
SYMBOL res 480 128 R90
WINDOW 0 0 56 VBottom 2
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SYMATTR InstName R1
SYMATTR Value 100k
SYMATTR SpiceLine tol=1 pwr=0.1
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WINDOW 3 32 56 VTop 2
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SYMATTR Value 0.5nF
SYMATTR SpiceLine IC=0
SYMBOL res 368 272 R90
WINDOW 0 0 56 VBottom 2
WINDOW 3 32 56 VTop 2
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SYMBOL res 64 128 R90
WINDOW 0 0 56 VBottom 2
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SYMATTR InstName R5
SYMATTR Value 100k
SYMBOL res -192 256 R90
WINDOW 0 0 56 VBottom 2
WINDOW 3 32 56 VTop 2
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SYMATTR Value 100k
SYMATTR SpiceLine tol=1 pwr=0.1
SYMBOL res 144 272 R90
WINDOW 0 0 56 VBottom 2
WINDOW 3 32 56 VTop 2
SYMATTR InstName R9
SYMATTR Value 5.1k
SYMBOL zener 208 400 R180
WINDOW 0 24 64 Left 2
WINDOW 3 24 0 Left 2
SYMATTR InstName D1
SYMATTR Value PTZ6_8B
SYMATTR Description Diode
SYMATTR Type diode
SYMBOL zener 176 432 R0
SYMATTR InstName D2
SYMATTR Value PTZ6_8B
SYMATTR Description Diode
SYMATTR Type diode
SYMBOL Opamps\\LT1056 -64 352 M180
SYMATTR InstName U1
SYMBOL Opamps\\LT1056 608 96 R0
SYMATTR InstName U2
TEXT -344 736 Left 2 ;Descrption: Bistable multivibrator circuit\nVersion: 0.1
TEXT -322 808 Left 2 !.tran 1m
\$\endgroup\$
1
  • \$\begingroup\$ Thanks. I changed the op amp to LT1056. The circuit hanged for too long with OP amp OP296. Now, I get the same signals. \$\endgroup\$
    – Amit M
    Commented Oct 14 at 14:36
2
\$\begingroup\$

As G36 mentioned, your circuit works if the OpAmps have enough bandwidth.

Using the very fast AD797 with 110 MHz bandwith the resulting frequency is 16.5 kHz, in the simulation from G36 using LT1065 with 5.5 MHz bandwidth the circuit generates around 8 kHz.

schematic

simulate this circuit – Schematic created using CircuitLab Triangle Generator

\$\endgroup\$
4
  • \$\begingroup\$ The frequency generated with LT1056 was about 9 KHz. \$\endgroup\$
    – Amit M
    Commented Oct 14 at 15:20
  • \$\begingroup\$ @AmitM Jens is trying to say that the opamp you originally chose, the OP269, is way too slow for this circuit. \$\endgroup\$
    – G36
    Commented Oct 14 at 18:47
  • \$\begingroup\$ G36,I also had seen that. Op296 Op07 are slow for this application. Op27 and OP37 are functioning. Thanks for your explanation. \$\endgroup\$
    – Amit M
    Commented Oct 15 at 2:33
  • \$\begingroup\$ G36, The gain bandwidth product of Op296 was about 450 KHz.The same for OP27- 8 MHz ,for OP37 -63 MHz , for L1056, it was 5.5 MHz. \$\endgroup\$
    – Amit M
    Commented Oct 15 at 2:43

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