Internal Structure of Op-Amps and Audio Power Amps

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nternal Structure of Op-amps and Audio Power Amps

On Internal Structure of Op-amps and Audio PoweAmps

l hereby express some of my personal opinions on how audio amps work. Don't confuse these with prop

eory, which is far beyond my grasp. I've never studied this field, it's only a hobby. I've written this text,

cause I've found out, that my intuitive explanations are often helpful to people who *do* study the theo

school.

nternal structure of op-amps and power amps

ost contemporary power amplifiers are conceptually quite similar to general op-amps, i.e. they posess tw

puts (inverting and non-inverting) and one output. The difference with power amps is, that their final sta

e estimated for higher output currents and, because maximum power dissipated in a given load is

oportional to square of voltage, the power amps usually have to stand higher power supply voltages.

metimes it may be difficult to distinguish the inverting signal input in an amp built of discrete devices,

cause we only need one explicit input for signal (the non-inverting one), and so the inverting input rem

mbedded in the schematic - but in most power amps it is identifiable if you follow the resistive divider in

gative feedback and if you're able to recognize the differential input stage.

l op-amps, including the monolithic ones, consist of several individual transistors. There are about two

ree main stages to an op-amp.

he first of them is a differential input stage, which basically has the task of deriving the difference betwe

e two inputs and usually doesn't have a high amplification just by itself. Then there's a voltage amplifica

age, which accounts for most of the op-amp's voltage amplification. The third stage is an optional currenoster on the output, which increases the op-amps load capacity. Some older small-signal op-amps

mpletely lack this stage, most contemporary genaral-purpose op-amps have one, and it's rather

pertrophic with all power amps.

efore I try to further explain each of these three blocks, I should explain some basic ideas of transistor

mplifiers.

rst of all, there are two basic ways of using a transistor as an amplifier.

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he first one, called the 'common emitter mode' is used mainly for voltage amplification, although it give

me current amplification too.

he base is driven by variable current and the collector is connected in series with a resistor, thus creating

riable voltage divider. If you want to drive the base with a voltage-based signal source, you have to bia

th a pull-up resistor to Ucc and AC-couple it with a cap to the previous stage. In order to achieve the be

nearity, SNR and amplification, you have to use particular values of resistors with respect to the transist

minal current amplification and noise specifications. You also have to take care not to overdrive the

mplifier. The amplification parameter (Beta, Hfe) may be quite variable even within a batch of single typ

ansistors, so that you may have to allow for individual setting of the bias current in each piece of equipm

tolerate distortion. For the reasons spelled above, this way of using discrete transistors as linear amplifyvices hasn't been employed for several years or decades already. Transistor structures in today's linear I

e to great extent self-biasing and easier to use in mass production than stand-alone discrete transistors. T

E.M. principle however remains in use within the monolithic ICs.

he second essential way of using a transistor is the Common Collector Mode (sometimes called simply a

ource follower', which is however a broader category).

has a high current amplification and is therefore usually used as a plain current booster. Its voltage

mplifiation is theoretically equal to one, in practise it's less than one. In some cases, there needn't even b

sistor in series with the base. As a demonstration, this way it should be possible to drive a small voltage

ht bulb (connected instead of the load resistor) with a TTL gate. (The TTL output alone wouldn't make

is not advisable to use C.C.M. with inductive loads - the circuit might oscillate or even blow out.

oth C.E.M. and C.C.M. can be used for switching purposes. In this case, with C.E.M. there's no need to

e transistor into linear mode.

ow that I've taken a quick glance at the function of a transistor, I can proceed to the main building block

ost op-amp and power amp schematics.

he constant current source is a good example.



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here's a voltage stabilizer in the base of the transistor. Thus, the resistor in series with the transistor's em

mply doesn't allow the I(ce) to rise over a certain limit, which is theoretically given by the Ohm's law (I

. As a result, the collector has a pretty flat U/I curve - at least much flatter than linear, even flatter thangarithmic - it's convergent to the theoretical limit for infinite collector voltage.

he constant current source acts as a high value resistor, connected to a high voltage. Such properties are

eful for setting bias current in final stages of the amp, and also for voltage amplification. Imagine a C.E

pe of circuit, with a constant current source instead of the collector resistor.

theory, such a circuit would have an infinite voltage amplification. Even in practice, the amplification i

gh that it's only possible to bias such a circuit into linear mode with a negative feedback.

nother important cornerstone of all op-amps is a differential amplifier. In practical schematics, it can be

ually easily identified as two transistors in the input area placed face-to-face close to each other, with th

mitters inconnected (either directly, or over small resistors). There are a few other devices around: two p

resistors in the transistors' collectors, and a single pull-down resistor from the interconnected emitters



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ound. Most practical differential amps however use a constant current source instead of the emitter pull

wn resistor.

he main purpose of a differential amplifier is to aggregate the two inputs of an op-amp (inverting and no

verting) into a single signal, which is passed to the next stages for further amplification. In fact, the two

puts are the bases of the two transistors to a differential stage. The output signal is drawn from the colleonly one of these transistors. As a result, one of the inputs is inverting, and the other one is non-invertin

he precise amplification of either input and the whole stage is to great extent unimportant, as we only ne

e differential stage to compare the inputs to each other. The amplification can be provided by the follow

ages (again, its precise value is unimportant, it only has to be real high) and the feedback takes care of th

st, so that in practice, the two inputs will always be real close to each other and will mostly function as

mparator. The negative feedback will struggle to keep them precisely balanced.

this point, let's take a look at some oversimplified schematics. The simpliest true op-amp schematic I'v

er seen consisted of six transistors. The following four drawings show a simple op-amp with four differ

hird" stages. (never tried to build this, it probably wouldn't work just like this)

the first schematic, there's no current booster at all.

only consists of the differential input (with a constant current source in the emitters) and a single voltag

in stage (featuring a single transistor in the C.E.M.).

he second schematic already contains an additional current booster, using a single transistor in the C.C.M



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he maximum load is determined by the pulldown resistor. (BTW.: Looks like Pure A Class, am I right?

diophiles around here? :)

he third schematic contains a push-pull (complementary) current booster stage, even with a non-zero bia

rrent.

he final stage in fact consists of two constant current sources standing back-to-back. The only difference

om regular CCS's is, that the ground of the two voltage references isn't tied to the output, but floating, so

at the CCS's are controllable by the output of the voltage gain stage.

his sort of current booster stages is quite common in small-signal op-amps.he fourth schematic shows another type of a push-pull current booster arrangement.



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he bias current can be set by the trimpot in base of the transistor titled "T". This sort of current boosters

ed in power amps and so the transistor "T" is sometimes thermocoupled to the very last output transisto

event the bias current from growing with temperature.

ote that in the last two examples, the resistor "R" is often replaced by another constant current source tocrease amplification, linearity and efficiency of the whole.

or an example of a truly working op-amp, take a glance at this schematic. This one is a bit more complic

an most others, because it's 'fully-complementary'.

you want to take a look at some practical op-amp schematics, go to the Philips Semiconductors' search

ge and type in "op-amp" as a keyword.

here's also an application note by Philips Semiconductors on op-amp theory. (BTW: Philips seem to hav

at appnote on virtually everything.)

eedbacks and system stability

he main op-amp concept closely resembles the general principle of error-driven control systems. You dr

e positive input with a refferential signal and you drive the negative input with an output signal of the

stem. If there's an instantaneous difference between the input refference and the output, the differential

put of the control system evaluates it as an error between the actual output state and the state desired, an

oves the output in an opposite direction than is the one of the error.

ith general control systems, this interpretation applies for various classes of input and output signals, an

stem designs. With op-amps and audio amps, the situation is quite simple: all the signals are contiguous

ltages, variable in time. Note that, although in the whole theory we only operate with three 'signals' (+in

out), we always end up needing a fourth signal to refference the rest to - this signal is the Ground. This

cause the voltage, processed by real op-amps, is a difference of potentials. Gimme a fixed point in the

iverse and I'll move the earth. (Anyone remembers who said that?) Even though in some cases we don'

ed the ground on the input, we always need something to connect the output load to. In my example giv

ove, we need the ground both on the input and on the output.

n ideal op-amp amplifies the difference between the two inputs with an infinite amplification ratio and w

zero response time. You can picture an op-amp as a comparator: if the (-)in is higher than the (+)in, the

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tput is -(infinity). If the (-)in is lower than the (+)in, the output is +(infinity). An upward motion of the

causes the output to climb high and vice versa. An upward motion of the (-)in causes the output to go lo

d vice versa. Or maybe I'd better say 'flip low'/'flip hi' as long as we work with ideal devices. I've got no

ea of how the theory deals with all these infinities, and in fact I don't care, as no ideal devices really exi

actical op-amps amplify the difference between their two inputs with a pretty high amplification within

asonable response time. Their input and output voltage range is limited by the power supply rails.

you connect the (-)in to the output and drive the (+)in with a reasonable signal, the output will exactly

llow the (+)in, because this way the feedback tells the op-amp to minimize deviations between the give

put and the output. Now imagine what happens, if you connect the (-)in to the middle point of a resistiv

ltage divider (e.g. a trimpot), that's connected between the op-amp's output and ground. This sort of a

edback network tells the op-amp to minimize error between the input signal (+in to GND) and the 'outp

gnal (output to GND) divided by the ratio of the two resistors in the feedback'. This way you can do all

things with the negative feedback, like setting it to various amplifications or making it frequency

pendent by including capacitors (or coils). You can even make the op-amp oscillate, which often happe

an unintended by-product of a miscalculated feedback experiment.

egative feedbacks are usually used in self-stabilizing circuits. Positive feedbacks, on the other hand, are

abilizing. You can achieve one by involving the (+)in in a feedback network. Such feedbacks are often u

introduce hysterisis in flip-flop circuits and comparators.troduction of frequency dependent devices into feedbacks makes quite a mess about the topic. These

vices not only cause the amplification to be frequency dependent, but also introduce phase shifts. Thus

me cases, the border between a positive and a negative feedback becomes rather fuzzy. Or I'd better say

at in such cases we have to work with all sorts of feedbacks within a single theory, that would allow us

ake some conclusions. Due to my limited education in electronics and math, I only know a little bit in th

ld, but I'll try to explain it.

here's a concept called the Nyquist's criterion, concerning system stability (in this case meaning

nvergence in time). It applies for control systems in general, not only op-amps and their linear feedback

tworks. Imagine a control system with a feedback (an audio amp is a good example of a negative feedb

stem). The Nyquist's criterion applies for the loop as a whole, i.e. in our example the external feedback

twork AND the amp itself. Both the external network and the internal devices of the amp introduce pha

ifts (in case of the amp I rather percieve it as a response time or delay). The overall phase shift is a sum

rtial shifts of the loop components. The overall amplification around the loop is a multiplicative produc

rtial amplifications (i.e. the gain in dB is a sum of partial gains). The criterion says, that the feedback

stem is stable, if at all frequencies, where the overall phase shift equals an integer multiple of 360

grees, the overall amplification is lower than one. I can say it the other way around, too: if you can f

frequency, where the phase shift is an integer multiple of 360 degrees, and the amplification is equal to

eater than one, the loop won't be stable. Unstability may either mean, that the output will wander off int

e of the limits forever, or that the system will oscillate. A positive DC feedback is an example of thermer, an oscillating system with a complicated frequency-dependant feedback is an instance of the latte

op-amp systems with a negative feedback, the 'negativeness' itself presents a 180 degrees phase shift. I

is example, please forget about the (+) input - it's just a DC biasing reference. Since it's impractical to ta

re of each individual integer multiple of 360 degrees separately, another approach is usually taken. Ima

e loop as an ideal amplifier and a series of low-pass filters (first-order RC integrators). The "ideal

mplifier" stands for the overall gain, and the low-pass filters represent the frequency dependent damping

ase shift. Now we've got 180 degrees in either direction to start with. There's no problem with the nega

0 degrees. There's however quite a puzzle with the positive direction. The easiest way to avoid the



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yquist's frontier is to add another low-pass filter into the already existing series. This low-pass filter mus

duce the overall gain to such extent, that at 180 degrees it's lower than one. This compensation is usuall

troduced by a local capacitor feedback across the stage that accounts for most DC amplification. The

ouble is, that this additional compensational filter stage eats another 90 degrees, so that we've only got 9

grees left for the remaining stages. Also, within 90 degrees, this additional filter stage has got quite a

mited steepness. As a result, in order to stabilize our amplifier, we either have to use pretty fast active

vices, or we have to cut open-loop amplification at higher frequencies, thus increasing distortion.

he graph is either drawn in the gain/frequency domain, or in the Z-plane. The former is often used with

dio amps, while the latter is used with general control systems. I'm not quite familiar with the latter, so

e the former. It suits us best, anyway.
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Internal Structure of Op-Amps and Audio Power Amps

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