I need to make a sawtooth oscillator. I'd prefer not to use the 555 to produce the ramp; I've tried several schemes and the results don't satisfy.
I'd prefer to use a chip 2206-like very much (I used it some year ago and the result was very good in term of THD).
I've read in the web that the 2206 has been obsoleted by the producer (Exar).
So I wonder, and asking you, if there's a good alternative to that chip which is as cheap and as easy to implement.
Sure, the RC charge curve is asymptotic, so it won't be linear.
I've found the constant current source works very well for making a linear sawtooth. Use identical PNP transistors, preferably from the same batch. You may even tape, epoxy, or shrink wrap them together to keep them at the same temperature.
I've tried this circuit http://www.allaboutcircuits.com/vol_6/chpt_6/8.html, getting the 2 pnp as in contact as possible. One of them is always saturated (Vce close to 0.1V or less). Is that normal ? Do you think I can trust that circuit ? Do you know a better one ? thanks and sorry for my ignorance on this matter.
It is better to post graphics as JPG or GIF files.
Just to be clear:
12V to pins 4 and 8
Ground to pin 1
Pins 2, 6, and 7 connected to C1
But you should be looking at the top of C1 for the sawtooth wave. All you'll see on the output is 12V interrupted by a very short downward spike. See my math below to see just how long that will take with the values selected.
The Control voltage pin should have something more like a 1nF capacitor. If you use a CMOS 555 timer, it may take a while for a 0.2uF cap to charge on startup, although after that it won't matter. For the simulation, you can leave it off.
100k and 10uF is a bit slow.
(12-0.7)/100k = 113uA
C1 must charge from 1/3 of Vcc to 2/3 of Vcc, so 4V differential.
V = It/C
t = VC/I
t = 4 * 113u / 10u
t = 45.2 seconds
In a real-world circuit, a generic aluminum electrolytic may have so much leakage current, that the circuit never charges.
This will be a linear function of the resistance, and an inverse linear function of the capacitance. So use a 0.1uF capacitor, and the period will be 0.452 seconds. With a 0.1uF capacitor and 10k, 45.2ms.
Hi polimorph, thank you for you patience. I successfully simulated using LTSpice too (see attach, png this time), taking output at C1. I didn't use the emitter resistor and the result seems good anyway. I'll implement the circuit soon...
thank you so much,
ah, another important question: which is the best way to create a conductive contact between the two transistors ? I've a conductive paste used for CPU, but I need to get the they as close as possibile ant avoid them detaching...
Real circuit OK!
I got 250kHz as I needed. Not thermal contact yet, emitter resistors, ~10mA overall consumption (all the circuit)... see attach. Result seems to me very good, but I'm not an expert so, polimorph do you think it is acceptable (apart the fact that I haven't created thermal contact) ?
The aim of this sawtooth is to feed a modulator to produce a PWM to make a very basic class D amplifier.