A capacitor is an electronic component, of which the primary purpose is to store an electrical charge electrostatically between two plates. Since a charged capacitor will block the passage of DC, (Direct Current.) capacitors are sometimes used as decoupling components where only the flow of AC (Alternating Current.) electricity is desired.

The many different types of capacitor are named by the type of insulation, known as the dielectric, used between their plates. This insulating material has a large bearing upon the electrical characteristics of the individual type of capacitor.

Electrolytic and tantalum capacitors are types of capacitor known as ‘polarised capacitors’. This means that they have to be connected into a circuit observing the correct polarity. Failure to do so may result in anything from the component’s eventual failure to its explosion.

A capacitor’s capacitance is measured in Farads, named after the discoverer of electrical capacitance; Michael Faraday:

Microfarad (uF, MFD) = 0.000001 of a Farad or 1 x 10 to the minus six of a Farad.

Nanofarad (nF)           = 0.000000001 of a Farad or 1 x 10 to the minus nine of a Farad.

Picofarad (pF)            = 0.000000000001 of a Farad or 1 x 10 to the minus twelve of a Farad.

- For example: 1000 pF = 1 nF = 0.001 uF.

If an individual capacitor has a voltage-rating printed on it then this is the maximum in-circuit voltage that it can operate at. Anything greater than that and the capacitor’s dielectric could rupture and become breached by an arc, causing the capacitor to explode in a worst-case scenario.

This article is unconnected to the 1980s group Frankie Goes to Hollywood.

A lot of people go out and throw their money away on alcohol, which ends up as part of the contents of the local sewer system rather quickly, on a Friday night, particularly in the UK; but I prefer to stay in and expand my mind: No, not with some new-fangled narcotic substance. – Rather; with study and experimentation in the style of geeky-relaxation:

This particular Friday night in July it was a bit damp outside anyway with somewhat lower than average night-time temperatures for the time of year, so unless I had something amazing planned at some external venue, which I didn’t, I was definitely going to stay indoors. There was some good TV on that evening too, so I’d spend a couple hours watching that and then relax in front of and away from the computer.

I was starting to get a little vacant-minded eventually, and I started to doodle. Now when most people doodle they draw physical things or swirls or patterns or shapes or something similar. When I start to doodle then I usually start drawing an electronic circuit diagram. – Honestly I kid you not. – It’s usually something fairly simple like a Hartley oscillator or a single transistor emitter-follower output stage; but very occasionally I become fully alert while I’m doing it, realise what I’m drawing, and suddenly an idea pops into my head from which I develop something else or it takes me onto another level mentally.

This Friday night was one such event: I’d been contemplating the Darlington transistor in a kind of semi-conscious state, and went on to remark to myself inside my head on the surprising number of hits I’d had on my article regarding a Darlington-pair amplifier circuit. Still in a dreamlike state I put that thought on hold and went on to imagine ways to mix a timebase signal with a direct current to produce an alternating current using a matched pair of bipolar power transistors. – That’s when I realised that I was doodling again; and I’d started to draw a matched pair of bipolar Darlington transistors configured as a high-gain audio amplifier.

I recoiled a little with a start: That was something I’d never thought of before, despite the concept staring me in the face. I thought it might be worth taking further while the idea was fresh in my mind. I started consciously working further on what had been my doodle: I added extra decoupling to the ground points, controlled variable simultaneous negative feedback across both Darlington pairs, 2 sets of potential dividers for biasing the Darlington bases separately…

After faffing about for a while and drawing a circuit diagram with so many corrections it was barely legible, I transcribed the circuit to a fresh diagram in order that it would be legible to anyone else… Then I decided to blog it.

So – fresh out of my mind, totally unrevised and untested, I present to you my idea for a single-channel monaural audio amplifier with gain controlled by means of negative feedback utilising a ganged potentiometer.

I think it’ll work, but I have no idea how well. It’s one of these ideas I draw up that I never actually build, and it remains a theoretical triumph of unstarted construction in my head to times unlimited. Here it is anyway: -

There seems to be an error in the diagram: It appears that I’ve drawn D2 the wrong way round.

If you’re qualified in electronics please feel free to criticise, critique, comment, other words starting with C; even build it and/or improve on the design if you like: ‘Your choice. (I deliberately left the circuit diagram small enough so that you could hopefully get it all in a single browser window in FireFox at a resolution of 1024 x 768 px.)

I didn’t choose any component values other than those of the 10 nanofarad capacitors across the base and emitter of Q1 and Q3: Including them like this does actually increase audio frequency response at bass frequencies. I heard about it somewhere ages ago and have actually tried it to prove that it works: It does; to a limited extent.

Having blogged that I’m now going to get a coffee and do something else. I’ll decide exactly what as I drink the coffee.

Tatty-bye for now.

Analogue amplification stages incorporating just a single active device, such as a transistor or op-amp, – as in a DC amplifier -  usually allow amplification of either the negative-going or the positive-going part of an AC waveform alone; basically cutting out half of the cycle and therefore causing immense distortion. There are two ways of overcoming this: Those being amplifying both halves of the waveform separately and recombining them at the output; perhaps at a decoupling transformer, (Too much copper wire, inductance, and consequential weight for my liking.) or by using two interconnected active devices to amplify both halves of the waveform at once, before passing the full AC waveform on to the next stage.

The latter method can be achieved with a push-pull amplifier.

Pictured below is a circuit diagram of a very basic push-pull amplification stage. (Please excuse the freehand drawing.)

The input signal; an alternating waveform, is fed via capacitors 1 & 2 to the bases of Q1 and Q2. The functions of C1 & 2 individually and collectively are two-fold: The first is that they individually shield the base connections of their respective transistors from any stray DC voltages from a previous stage, also they separate any DC potentials present at the base junctions of Q1 & 2; therefore preventing unintentional and accidental biasing of one another.

Resistors 3 & 4 act as a potential divider biasing the base of Q1 to 0V7. Resistors 3 & 4 have the a similar effect effect on the base of Q2: However since Q2 is a PNP transistor, the values of resistor used in the case of the R3,4 pair are reversed; therefore giving the base of Q2 a negative bias with respect to that of Q1.

Preset potentiometers PR1 & 2 set the potential of the transistor-pair with respect to the supply rails; and consequently the swing-maximum of the output-waveform. Resistors R5 & 6 are a precaution to avoid the peak output level colliding with the supply voltage and therefore causing distortion.

Capacitor C3 provides AC decoupling for the transistor pair.

Note that the emitters of the transistors are connected together and the output taken from that connection. This allows the inclusion of the traditional collector load resistance in both cases.

A waveform appearing at the input flows through both C1 and C2 to the base of the individual transistors. If the waveform is on the positive-going half of the cycle it lowers the conduction of Q2 and raises the conduction of Q1. If the waveform is on the negative-going half of the cycle the reverse occurs: Hence the output polarity mirrors the input polarity to whatever degree of amplification is involved.

A Little Background Information:

Prior to the advent of digital electronics, this type of circuit configuration was widely used in analogue receiving and amplification devices throughout the 1950s, 60s, and to some extent even the 70s. Back in the 1950s before the transistor became widely used in electronic circuitry, they would use a pair of triode or pentode  thermionic valves in the place of the transistor pair. As technology developed the manufacturers developed smaller valves with two triode or pentode sections for the purpose, screened from one another by a metal electrode.

(Example: ECC82 (European Nomenculare), or equivalent 12AU7 (American nomenculare.) AF double-triode with a 6.3 Volt heater supply. The European equivalent with a higher heater voltage was the UCC82 which required a 32 Volt heater supply.)

Valves were also manufactured with a pre-amplification or oscillator stage included, usually a triode; along with a main amplification device, usually a pentode. (Example ECL85 (6.3V heater supply), UCL85(32 V heater supply), and PCL85 (17.5 Volt heater supply, commonly used in the audio output stages of televisions. – Right up until around 1973.))

During the late 1960s/early 70s, valves began to be excluded from the designs of electronic devices, in preference for the transistor; which was lighter, lasted a lot longer, required less voltage to function, and didn’t require an internal heater powered from an external source to make it work.

(There was a period at the very end of the 1960s which lasted a little way into the 70s where equipment manufacturers would produce valve/transistor hybrids, especially in the case of televisions. These exhibited a few benefits over valve-only technology; such as they took less time to warm up before they started working, and the amount of mains-hum distortion was reduced to a large extent.)

There: A free history-lesson along with the main subject. There’s value-for-money; even though there was no charge in the first place.

Personally Speaking:

Just in case you’re wondering, I do just about remember those old days mentioned; especially the latter valve/transistor technology. I was just getting into electronics in those days. – And yes I was rather young to be messing about inside televisions et al. I practiced hobby electronics until fairly recently, when I got qualifications in the subject after a crash-refresher course at college. I became interested in computers in the late 1970s, around the time the Commodore Pet was released to market. Upon leaving school I followed the arts for a while, at the same time as running a small hobby-enterprise in analogue electronics, until I got back into both computers and digital electronics in the late 1990s.

(Oh yes. – Just in case you were wondering; I do remember the large B9D-base line-output pentode valve used in some televisions right into the 1980s; although I can’t remember the alphanumeric designation offhand. It started with a “P”, but that’s pretty obvious. – Most television valves did, other than maybe the HF triode-pentodes in the UHF/VHF tuners of some 1960s models. – Apart from some of the B8A valves of the early 1960s with the metallic base. – Now that is going back a bit too far.)

Ah I just remembered: PL504.  

. . . . . . . . .  . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .

I was just proof-reading after writing this lot and I remembered; I need to hyperlink. There is so much I should hyperlink. If this article takes longer than expected to produce then that’s part of the reason why.

Oh wow; this’ll be fun!

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A transistor is an electronic component. It is defined as an "active" electronic component because it is able to perform more than one function at more than a single level.

The most basic of transistors is the bipolar transistor. It has three connections; those being the base, collector, and emitter. A circuit-diagrammatical representation of the bipolar transistor is shown in Fig.1 below. The collector is the connection at the top, the base is the one in the middle, and the emitter is the one at the bottom with the arrow on it. In addition to the transistor’s amplification factor, the base and emitter act as a diode. (See Fig.2 (i) and (ii).)

The direction of the arrow indicates whether the type of bipolar transistor is NPN or PNP. (Which stands for Negative Positive Negative or Positive Negative Positive.) The difference amounts to the way that the transistor is connected in a circuit with regard to the DC polarity. This polarity is caused by the transistor’s substrate layers being doped with a P-type and an N-type substrate. (See table of links.)

Fig.3 shows a PNP transistor connected into a basic circuit. Fig 4 shows an NPN transistor connected into an equivalent circuit. The resistors in the circuit limit the current flowing through the device and set the device’s voltage potential point with respect to the supply rails. The capacitor drawn in with dotted lines is a decoupling capacitor which, along with R3, decouples the collector (PNP) or the emitter (NPN) to ground; limiting distortion in the output and/or compensating for any residual ripple present in the supply rails – depending upon its value combined with that of R3 giving a certain AC reactance. (A subject beyond the scope of this article.)

Resistor, R1, is connected as a DC current-limiting resistor in both cases, to the base of the transistor; limiting the base current which in normal operation should not rise above approximately 1/10th of the current flowing between collector and emitter. (As low as 1/100th is the preferred quiescent value for maximum amplification in most high-gain devices.) The differential between the two sets the transistor’s working amplification factor or beta. This is limited by the actual electrical characteristics of the chosen device itself.

This article cannot hope to go into the full details and various functions of the bipolar transistor under all conditions, and even the AC amplification operations of said device are far too in-depth to discuss in the space allocated.

For further information on this device please visit links in the table of links below.

Table of Links:

Connecting two transistors as a Darlington pair by connecting the emitter of the first transistor directly to the base of the second transistor multiplies the beta of the first transistor by the beta of the second transistor to give an extremely high-gain device.

Imagine, for instance; two NPN devices, each with a maximum gain of 50, connected in such a way, giving a device with a maximum gain of 2500: This would be useful for boosting the output of a piezo-microphone for example; notorious for its low output.

In the basic circuit-diagram of a Darlington amplifier below; the input is DC decoupled by C1, resistors R1 and 2 form a potential divider biasing the base of Q1 at exactly 0.7 Volts, R3 is the load resistor on the collector of Q1 – which drives the base of Q2; R4 restricting its collector load, and the R4/C2 combination decoupling its emitter to ground.

Since the base of Q1 is at 0.7 Volts, both Q1 and 2 will be in an “always on” state, and sensitive to any tiny ripple passing through the input capacitor C1.

Bearing in mind that the input ripple will probably be of only a few microamperes; the R1,2 pair should be selected with as high a resistance as possible – Within the megohm range to limit the current already present on the base of Q1 to a fraction of a microampere if at all possible.

Q1 should be chosen such that its base current need only be negligible for it to respond. With a beta of 50 the resistance of R3 should be within the range of around -2 to -40 times that of R1, so as not to drive the transistor into saturation.

Again, having a beta of 50; Q2 should be run ideally at between 2 and 40 gain.

Suggested component values to run the circuit at a voltage of 1.5 Volts are as follows:-

R1: 2M2, R2:(1M with 250K Lin. Preset in series.) R3: 100K, R4: 22K, R5: 110R

C1: 1uF 10V Elect., C2: 100uF 10V Elect., C3: 10uF 10V Elect.

Q1: BC107B, Q2: BC109C

Note: I haven’t built this circuit myself; and it’s been drawn up for demonstration purposes only: It’s very basic and wouldn’t give brilliant sound quality anyway, but should nevertheless “work”.

Today we’ll look at a very basic analogue audio-amplifier circuit and examine how the principle of negative feedback can reduce distortion:

Capacitor, C1, decouples the input to potential divider, R1&2, keeping the base of Q1 at 0.7 Volts – Therefore any AC ripple will be picked up instantly by Q1 and amplified. C2 is the first example of negative feedback: With a value of a nanofarad, it feeds higher frequencies from the collector of Q1 back to its base, thus giving a better response to bass and mid-range audio-frequencies than treble notes. Q1’s Iceo is set by the combined resistance of R3 and R4, and decoupling is provided by electrolytic capacitor C4 to ground. (C4/R4’s reactance should be taken into account when deciding the component values.)

The first stage is decoupled from the following stage by C3, and resistors R6 and R5 act as a potential-divider to set Q2’s base at 0.7 Volts. The output is taken from Q2’s collector via C6.

Let’s assume that the beta for each individual transistor is 100, and that we’ve chosen components that will allow each transistor to operate with a working beta of 50: Therefore a tiny signal of 10mV p-p applied to the input will give a massive output of 1 Volt p-p. – That’s quite some amplification; and not only will the clean signal be amplified 100 times but so will any distortion on the input waveform. Second to that, if we’re amplifying an audio signal with variable amplitude on the input; there are going to be occurrences where this circuit; particularly Q2, is driven to saturation – A 2.1 Volt p-p signal from a preamplifier at the input, for example, producing an output of 20 Volts p-p with a flattened crest where Q2 saturates – Thus producing distortion.

To compensate for the above unwanted distortion we introduce negative feedback:

With the introduction of PR1 and C7 we have allowed a regulated path back to the input from the output, DC decoupled by C7: Therefore the higher the resistance of PR1 the less the entire two-stage circuit is bypassed; therefore the more current flows through Q1 and 2 and is amplified.

This is a similar function to that of R2 in the basic DC inverting operational amplifier circuit below:

Again we note that should the voltage on the inverting input at the virtual-earth junction of R1/2 reach a certain level with respect to the supply voltage it will cause IC1 to saturate. R2 gives negative feedback between input and output, therefore limiting this effect. 

In very basic terms then, the function of negative feedback is to reduce overall gain, thereby limiting distortion.

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