What types of capacitors are these?

Howdy folks,

I am building an oscillating circuit that is coupled to an amplifying transistor through a capacitor. The circuit averages about 1 MHz, so I want to use a "fast" capacitor. So far my knowledge of capacitor parameters has been limited to their capacitance, and the voltage they can withstand. I have read of capacitor "speed", but know little about it. I understand that a cap's speed is determined by it's capacitance and construction.

The portion of my circuit surrounding the capacitor looks like like the drawing below. The oscillator is on the left, and the transistor on the right is an amplifier. The capacitor in question is in the middle of the drawing, and is identified by C:

The four capacitors I have on hand that meet the capacitance and voltage specifications are these:

Can anyone tell me what kind of capacitors these are, and what is the relative "speed" of those types?

Thank you!
Chris

Without seeing specs it is difficult to say which ones are which with confidence:
Left most is probably polystyrene
Next is probably ceramic
Third is probably a film capacitor but can’t be sure.
Fourth probably ceramic, maybe stacked.

All real caps have errors and this error is usually referred to as tan, or how strictly the capacitor under test holds to 90 degrees phase shift. The errors in tan are caused by internal resistances or ESR and inductances.

Basically, the polystyrene and film caps will be better, but they are much bigger than ceramics and electrolytic types.

For this problem you need to consider what that capacitor is doing. Basically it is a coupling capacitor that is it is transferring a signal.
Each type of capacitor has a different inductance and loss. The inductance determines what it stops being a capacitor and acts more like an inductor. The loss, as the name implies, determines the amount of signal it absorbs.

This will explain the differences Capacitor types - Wikipedia

ChrisTenone:
I have read of capacitor “speed” …

Where?
I’d be interested to see if whoever wrote it knew what they were talking about.

“Speed” is not a parameter of a capacitor.

Any of those (that davetcc is probably correct in identifying) will work just fine, you will not be able to tell the difference between them.

However, at higher frequencies (say, 100MHz) the type and construction of a capacitor becomes a lot more important. Any capacitor is not a pure capacitance, it also has inductance which gives rise to a changing impedance with frequency.

You could say “a capacitor with a relatively high inductance would not work well at higher speeds”
but an engineer would not say that, instead they’d say “such a capacitor would not work well at higher frequencies”.

For larger values of capacitor (commonly >10uf), the non-ideal nature is often measured as ESR (equivalent series resistance) which is very, very roughly the ‘speed’ at which the capacitor can charge and discharge (to smooth a power supply for example).

So, as well as nominal capacitance and voltage rating, a capacitor may have ESR and/or impedance specified - not just single values, but graphed against frequency.

You might need an “RF capacitor” or a “Low-ESR capacitor”, but not a “high speed” one.

Yours,
TonyWilk

ChrisTenone:
I have read of capacitor "speed", but know little about it. I understand that a cap's speed is determined by it's capacitance and construction.

Perhaps you are talking about capacitive reactance?

1MHz is a relatively low frequency, pretty much any type of capacitor will work as a coupling or decoupling capacitor I suspect.

The key spec is the self-resonant frequency of the capacitor, some datasheets show this (though note that
layout can affect this frequency significantly).

Differentially measure the drop across the coupling capacitor.
If significant, try a different value and/or type.

.

You might find that the low input impedance of the second stage loads the oscillator too much.

The left most device is a hermetic something or the other, probably a tantalum. That would be the most long term stable cap you have due to its packaging.

Just like resistors, capacitors have a temperature coefficient, the TCC. Some parts have the tempco as part of their marking, the ceramic disc shows Z5U which means it has a usable temperature range of +10 to +85 C with a tempco of +22,-56% change over that range. Not exactly precision devices but good enough for HF interstage coupling and bypassing.

What's the 10k resistor between the collector and emitter of the second transistor for?

And the 1M would be better placed between the collector and base of that transistor - makes it less affected by it's beta in setting the bias......

Allan

This is why this place rocks: You've told me more about capacitors than I thought there was to know.

Thank you all!

Davetcc, thanks for such a direct answer. I will be using capacitor b.

Grumpy_Mike, I read the wiki article, but it didn't give specific information on the appearance of the various capacitors. However on re-reading after your post, I confirmed that b is indeed a ceramic capacitor - it looks like ceramic! (especially with that chip in the disc... :stuck_out_tongue: )

TonyWilk, I know I saw this capacitor referred to as "fast" somewhere, but I cannot find it now. I stand corrected, and will no longer refer to the "speed" of the capacitor. The term capacitive reactance (per dougp) seems to describe the property I am thinking of, however as stated by MarkT, my paltry 1MHz does not challenge any modern capacitor.

"Dammit Larry! I'm a chemist not an engineer!" - What, exactly do I have to measure? And how?

aarg - what would be the symptoms of overloading the oscillator?

avr_fred - The little cans are tantalum caps! Cool - I have a bunch of those. I thought they were polarized, and considered the side with the bump to be positive. Thanks for the ID. It makes sense.

allanhurst: that resistor is the bottom half of a voltage divider.

Here is the completed circuit (an instance of Aaron Logue's tunneling noise white noise oscillator.) as you can see, I used the disc-shaped, ceramic cap:

"Dammit Larry! I'm a chemist not an engineer!" - What, exactly do I have to measure? And how?

Change your paper chemistry filter when it doesn't let fluid go through :wink:

I assume you have access to an oscilloscope:

Measure the 1 MHz signal on the input side of the capacitor, lets say it is 1Vpp.

Measure the 1 MHz signal on the output side of the capacitor, lets say it is 0.1Vpp.

Install a larger capacitor.

Measure the 1 MHz signal on the output side of the capacitor, lets say it is 0.9Vpp.

The coupling capacitor size is probably okay now.

.

ChrisTenone:
Here is the completed circuit (an instance of Aaron Logue's tunneling noise white noise oscillator.)

I think that's the one that changes over time; something about wear-and-tear. If it is, the whitening algorithm used for atomic decay is the best choice. (That is, if you plan to use it for generating random numbers.)

larryd:
Change your paper chemistry filter when it doesn't let fluid go through :wink:

I assume you have access to an oscilloscope:

Measure the 1 MHz signal on the input side of the capacitor, lets say it is 1Vpp.

Measure the 1 MHz signal on the output side of the capacitor, lets say it is 0.1Vpp.

Install a larger capacitor.

Measure the 1 MHz signal on the output side of the capacitor, lets say it is 0.9Vpp.

The coupling capacitor size is probably okay now.

.

Thanks Larry! I will do just that.

Yes, I have read that as well, and it fascinated me. Tunneling as you know, is the process of an electron moving through an insulator without actually being in the insulator. That is, it disappears from one location, and appears in another without occupying any space in between. Similar to how electrons move from one energy level to another in an atom, emitting a photon equivalent to the energy difference in the process.

Now when you hook up two transistors back to back this way, the electron tunnels from one (the reverse biased one) toward the other. However, it damages the junction in the process, suggesting to me, that it was present in the middle during tunneling.

I am generating random numbers (I built a dungeon dice roller using my circuit), but my ulterior motive is to produce damaged NP junctions, and then look at them with an electron microscope and microprobe. I'd like to see what tunneling damage looks like.

Excellent! We expect pictures!

ChrisTenone:
I am generating random numbers (I built a dungeon dice roller using my circuit), but my ulterior motive is to produce damaged NP junctions, and then look at them with an electron microscope and microprobe. I'd like to see what tunneling damage looks like.

Interesting !

is that why you went for TO-18 (tin can) transistors then ?

Yours,
TonyWilk

TonyWilk:
Interesting !

is that why you went for TO-18 (tin can) transistors then ?

Yours,
TonyWilk

Yes, I figured the substrate would be easier to get at, but I've never opened one of those cans.

Well, that and they look cool and sciency. I put an oversized heat sink on the voltage regulator, just so the circuit would look badass.

Tunneling as you know, is the process of an electron moving through an insulator without actually being in the insulator.

Not quite, it is electrons passing through a barrier that you not expect because that barrier is higher than the energy in the electron. Typically the barrier is caused by a PN junction in the silicon. Not really anything to do with insulators.

Grumpy_Mike:
Not quite, it is electrons passing through a barrier that you not expect because that barrier is higher than the energy in the electron. Typically the barrier is caused by a PN junction in the silicon. Not really anything to do with insulators.

I guess I'm thinking of an electrochemical environment that does not have available conduction bands. That kind of insulator, not like a dielectric layer. I've learned a new engineering term here - barrier.