Calculating R, C & maximum input current of Oscillator Circuit

Hymie

Joined Mar 30, 2018
1,277
V1 and V7 vary in sync, V1 sets the frequency of the output ramp wave by sending a reset pulse, the speed at which it is reset depends on the timer value sent by a microcontroller, (connectect to a midi device). subsequently, the V7 is set so that it charges the capacitor at a rate so that an amplitude of 5v is produced just before reset is sent to V7. Therefore the output amplitude Is at a constant 5V, even when the frequency varies. My design above already has 2 free TL084 on the IC, so if there is no downside to using them. Then I guess it would make sense to save space no? also, is the LM386 just a IC of the LIN topology you displayed? if so, would it not make sense to use this ?
Yes, it makes perfect sense to use one of the spare op-amps within the package for the audio amplifier section.

On a practical note, it might be worth configuring the 4th unused op-amp within the package as a unity gain buffer (with no input signal) by shorting the output to the inverting input. Otherwise with two floating inputs, combined with its open loop gain could result in unstable operation due to stray voltages.
 

Hymie

Joined Mar 30, 2018
1,277
Yes, it makes perfect sense to use one of the spare op-amps within the package for the audio amplifier section.

On a practical note, it might be worth configuring the 4th unused op-amp within the package as a unity gain buffer (with no input signal) by shorting the output to the inverting input. Otherwise with two floating inputs, combined with its open loop gain could result in unstable operation due to stray voltages.

With your circuit design ensuring that the ramp is to 5V, then the amplifier section only needs a gain of 3.6 for the output to be able to drive rail to rail (+/-9V)

Best make it 5 to give some wiggle-room.
 

Thread Starter

Jack Tranckle

Joined Jan 20, 2016
73
Regarding the values of C1 and RS2. I have experimented with different variations of RC values to see what happens (with rc = 220u). The resistor value can go down as far as 90Kohm, (with C1 = 2.22nF) before there is an error. What is it that is limiting it from going lower? The current at this point is 2.6uA. In the opposite direction, there is no error at all. I tested R = 1000kohm, C1 = 220pF and it simulated fine.

2222.PNG
 

Hymie

Joined Mar 30, 2018
1,277
Regarding the values of C1 and RS2. I have experimented with different variations of RC values to see what happens (with rc = 220u). The resistor value can go down as far as 90Kohm, (with C1 = 2.22nF) before there is an error. What is it that is limiting it from going lower? The current at this point is 2.6uA. In the opposite direction, there is no error at all. I tested R = 1000kohm, C1 = 220pF and it simulated fine.

View attachment 149990
The limit might be due to the available current from voltage source V7, or alternatively as C1 increases in value, transistor Q1 is unable to fully discharge the capacitor (either due to the period it is switched on, and/or the maximum current it can pass).

If the latter – then I would expect the op-amp output ramp not to return to 0V (make sure that the scope is set to DC coupled and not AC coupled).

The maximum TL084 op-amp output current can be a low as 10mA; with a constant current charging a capacitor, the voltage across the capacitor is given by V = (I x t)/C. By substituting your circuit values you can determine whether the maximum slew rate of the op-amp (due to the capacitor loading) if affecting the result – but I would not expect this to become an issue until C1 is of the order of a uF.
 

Hymie

Joined Mar 30, 2018
1,277
For additional brownie points from your lecturers’ you could show your working in relation to the design of the amplifier stage.

With +/-9V rails, the maximum (non-clipped) rms sine wave voltage will be 9/root 2 = 6.36V.
Feeding this into an 8 ohm speaker will give a power output of 5W.

The transistor driver stage is configured as a class AB amplifier (with the op-amp providing the drive and biasing). Class AB amplifiers have a theoretical efficiency of around 75% - therefore the power losses in the transistors (at full volume) will be slightly less than 2W (1W per device).
Your chosen transistors, each needs to be capable of dissipating this power.

As an alternative, you could calculate the maximum rms voltage of your amplifier output waveform (since it will not be a sine wave), and use this voltage in the above calculations to determine the output transistor power dissipation under full power output.
 

Hymie

Joined Mar 30, 2018
1,277
The limit might be due to the available current from voltage source V7, or alternatively as C1 increases in value, transistor Q1 is unable to fully discharge the capacitor (either due to the period it is switched on, and/or the maximum current it can pass).

If the latter – then I would expect the op-amp output ramp not to return to 0V (make sure that the scope is set to DC coupled and not AC coupled).

The maximum TL084 op-amp output current can be a low as 10mA; with a constant current charging a capacitor, the voltage across the capacitor is given by V = (I x t)/C. By substituting your circuit values you can determine whether the maximum slew rate of the op-amp (due to the capacitor loading) if affecting the result – but I would not expect this to become an issue until C1 is of the order of a uF.

Forgot to add; make sure that you are looking at the output of IC U2A, and not post capacitor C3 (where the scope is shown connected in your original post).
 
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