triangular wave generator help

[erictham83]

For this circuit, if i try to amke the oscillator operate at 20kHz and the output swing btw +/-5V where the output current i want to be below 100mA with the resistor value >1MOhms is undesirable. How can i calculate:
i). Ri and C1?
ii). minimun working voltageof capacitor
iii).ratio of R1 to R2 to make the comparator swing when the v1 is +/-0.5Vcc

Can anyone teach me how to calculate this as this is one of the question in my past exam paper.

 

[Roff]

For this circuit, if i try to amke the oscillator operate at 20kHz and the output swing btw +/-5V where the output current i want to be below 100mA with the resistor value >1MOhms is undesirable. How can i calculate:
i). Ri and C1?
ii). minimun working voltageof capacitor
iii).ratio of R1 to R2 to make the comparator swing when the v1 is +/-0.5Vcc

Can anyone teach me how to calculate this as this is one of the question in my past exam paper.

Your questions are not clear.
If the output is +/-5V, does that mean that VCC=10V?
What is the 100mA output current referring to?
Does the comparator output swing from +vcc to -vcc?

 

[erictham83]

Sorry maybe i not mention clearly. The oscillator is operates at 20kHz and the output wave is swing between +/-5V.
and given that the op-amps operate best with output currents below 100mA.
The resistor value >1MOhms is undesirable.

This is all the information i get from the question.

 

[Roff]

You said, "output swing btw ±5V" and " v1 is ±0.5Vcc".
I conclude that this means vcc=10V.
Arbitrarily set Ri=10k.
VN=0 (Do you understand the concept of "virtual ground"?)
Do you understand that, in an op amp, input current≈0? (This is important.)
If you assume the comparator output swings rail-to-rail (±10V), then I(Ri)=I(C)=±10V/10k=±1mA.
Since you want F=20kHz, and T=1/F=50usec, then T/2=25usec.
A capacitor charging at a constant current charges at the rate dv/dt=I/C, or C=It/V. We know that we want the cap to go from -5V to +5V in 25usec, so C=1mA*25usec/10V. Therefore C=2.5nF.
The comparator will switch each time its input voltage reaches zero volts. When the comparator output is +10V, V1 is going negative. When it reaches -5V, the comparator input must be zero volts. Therefore, R2/R1=2.

 

[erictham83]

Ya, i understand the concept of virtual ground. and thank Ron_H...u give a great explanation and i had understand it now. Thanks again for urs help.