Upper frequency limit of an oscillator

[Akshatha Venkatesh]

I have an astable multivibrator circuit , which operates as expected only upto a certain frequency. To increase the oscillation frequency, if I increase the base resistors or decrease the capacitors , the oscillation completely stops. What could be the reason ? How do I go about analysing what the maximum frequency could be from the multivibrator output?

 

[Harald Kapp]

It certainly helps if you post a schematic of your circuit and tell us
- up to which frequency does the oscillator work?
- what is the frequency you want to achieve?

 

[Akshatha Venkatesh]

It certainly helps if you post a schematic of your circuit and tell us
- up to which frequency does the oscillator work?
- what is the frequency you want to achieve?

I want to achieve around 300kHz, but for the above values there are no oscillations, for 150k , I'm getting about 180khz.

 

[Harald Kapp]

Your capacitors are very small. Usually larger capacitors and smaller resistors are preferable to provide enough base drive current to the transistors. Check this website which has a useful table of suitable compoent values.

 

[BobK]

if I increase the base resistors or decrease the capacitors , the oscillation completely stops.

You would decrease the resistors to increase frequency.

Bob

 

[Akshatha Venkatesh]

Your capacitors are very small. Usually larger capacitors and smaller resistors are preferable to provide enough base drive current to the transistors. Check this website which has a useful table of suitable compoent values.

Take a look at my circuit and calculation below, if I reduce the resistance, I can't get the oscillations. I also simulated the above circuit with the same component values in LTspice, it works.

 

[Akshatha Venkatesh]

You would decrease the resistors to increase frequency.

Bob

If I decrease the resistance, there's no oscillation

 

[BobK]

That's not what you said before, you said if you increase the resistance it would stop oscillation. So only 1 value of resistance oscillates? I guess you were lucky to find it.

Bob

 

[Akshatha Venkatesh]

That's not what you said before, you said if you increase the resistance it would stop oscillation. So only 1 value of resistance oscillates? I guess you were lucky to find it.

Bob

Take a look at my calculation

 

[Harald Kapp]

I also simulated the above circuit with the same component values in LTspice, it works.

A simulation is only a limited image of reality.

Take a look at my circuit and calculation below

The calculation is correct in terms of getting a value for C in terms of f and R. However, you'll have to take into account the physical behavior of the components, too. With a base resistor of 140 kΩ and a supply voltage of 5 V you get a base current into the transistors of Ibase ~(5 V-0.6 V)/140 kΩ = 31 μA. Assume a gain of 100 for the transistor, then you get a collector current of Ic = 100 * 31 μA = 3.1 mA. 3.1 mA will drop Vdrop = 3.1 mA * 500 Ω = 1.55 V.
The transistor's collector emitter voltage will be Vce = 5 V - Vdrop = 3.45 V. The transistor is not in saturation.

But the derivation of the equations you used assume that the transistor are in saturation, Therefore you need to decrease the base resistor's values or increase the collector resistor's values.

 

[Akshatha Venkatesh]

But why assume gain as 100 ? And when I increase the collector resistor values , it doesn't work , no oscillation. If I increase Rc, Ic would decrease , in-turn Ib would decrease and hence the transistor stops switching on. This was my reasoning. Could you please explain where I'm wrong.

 

[Harald Kapp]

But why assume gain as 100 ?

Because that is a typical value for small signal transistors. Look up the datasheet of the transistor you use to get a better value. Note that the gain can vary widely.

If I increase Rc, Ic would decrease , in-turn Ib would decrease and hence the transistor stops switching on.

No. If you increase Rc, the same collector current will create a higher voltage drop across Rc, thus less voltage drop across the transistor. Saturation is an operating mode of a bipolar transistor where Vce < Vbe (see here).

What specific type of transistor are you using? It may simply not be suitable for 300 kHz operation.

 

[Akshatha Venkatesh]

Because that is a typical value for small signal transistors. Look up the datasheet of the transistor you use to get a better value. Note that the gain can vary widely.

No. If you increase Rc, the same collector current will create a higher voltage drop across Rc, thus less voltage drop across the transistor. Saturation is an operating mode of a bipolar transistor where Vce < Vbe (see here).

What specific type of transistor are you using? It may simply not be suitable for 300 kHz operation.

BC846B- NPN general purpose transistor. How do I determine if a certain transistor is suitable or not for 300kHz operation.

 

[Harald Kapp]

The BC846B has a DC current gain between 200 and 450.
The transit frequency (where gain passes below 1) is >=100 MHz, therefore 300 kHz should pose no problem.

Are you sure you have all connections correct?

 

[Akshatha Venkatesh]

The BC846B has a DC current gain between 200 and 450.
The transit frequency (where gain passes below 1) is >=100 MHz, therefore 300 kHz should pose no problem.

Are you sure you have all connections correct?

Yeah , the connections are right.

 

[Akshatha Venkatesh]

Yeah , the connections are right.

General astable multivibrator connections

 

[Harald Kapp]

Did you double check the pinout of the transistors?

 

[BobK]

From playing around in LTSPICE, you need much lower collector resistors. Try 100Ω.

Highest I could get is about 1.2MHz using the following:

2N3904

Rc: 77Ω
Rb: 5K, 56K (they have to be different for it to oscillate in LTSPICE)
C: 400pF

Bob

 

[Akshatha Venkatesh]

Did you double check the pinout of the transistors?

Yes

 

[Audioguru]

Maybe it is built on a solderless breadboard and its stray capacitance between all the rows of contacts and wires and inductance of its many long wires prevent the very low value capacitors and very high value base resistors from working properly. Make it with a compact pcb layout using capacitors and resistors with reasonable values as shown in the article.