transformer coupeled power amplifier

[Disha Karnataki]

our teacher taught us about transformer coupled power amplifier with the transistor in ce configuration, while noting down the advantages of this kind of amplifier she mentioned that the use of transformer helps in
1)impedance matching
2) to couple two stages i.e amplifier and the loud speaker.
disadvantages was: THAT CE CONFIGURATION CAN'T BE USED DIRECTLY(i.e to connect to the load) BECAUSE IMPEDANCE MATCHING CAN'T BE ACHIEVED. i didnot get this point as to why impedance matching can't be achivied??? the answer given to this question was because of high o/p impedance of ce configuration.
i want an elaborate ans to this sentence please explain me this....
SO, inorder to overcome this limitation cc configuration is used where transformer isn't required for impedance matching.. was the final ans given to overcome limitation caused by ce amplifier.

 

[chuckey]

I think your lecturer has combined two different problems. For audio frequencies you can use any class of amplifier you want, they can all work to the same specification, some classes are extremely inefficient i.e. 20 watts in for every watt out, so the amplifier gets hot and needs some method of getting rid of the heat - fans and big heatsinks.
The problem comes with radio frequencies when you are trying to deliver power to a remote load via cables. The cables contain parasitic capacitance and series inductance, so if the cable is long enough, the ratio of the voltage on it divided by the current, give a characteristic impedance in ohms of say 50 -100 ohms. If both the load and the amplifier do not present the right ( characteristic) impedance to the line, the line turns into a tuned circuit and different frequency go through it with different losses.
Frank

 

[crutschow]

Audio power amplifier impedance matching is only used with the old tube (valve) amps which have a high voltage, high output impedance, and low output current. For that you need an impedance matching transformer to efficiently couple the tube output power to the low impedance speaker.

This matching is not needed with solid state amps since they naturally have a relatively low voltage, high current output. The effective output impedance is made very low (typically less than an ohm) by negative feedback. Output transformers are large, expensive, and add distortion so they are not used, if it can be avoided. And thus they are not commonly used with solid-state audio power amps.

 

[Audioguru]

Your teacher was teaching about transistor audio amplifiers made 58 years ago.
Here is a low power (only 50mW) transistor amplifier made by Philips in 1955 that used a transformer to match the fairly high common-emitter output transistors to the low impedance speaker. Since the transistors were all PNP germanium ones the battery polarity is shown wrong:

 

[Disha Karnataki]

yes exactly this is what she was teaching this is called as push-pull amplifier..
see at this https://www.google.co.in/search?q=t...tutorials.ws%2Famplifier%2Famp_5.html;400;308
later she taught us the second amp ckt where ce configuration was replaced by cc configuration in order to remove the use of transformer in the ckt as here:https://www.google.co.in/search?q=t...nx.org%2Fcontent%2Fm32262%2Flatest%2F;440;315
u have mentioned that inorder to match the high emitter o/p to the low impedance of the loud speaker(LS) v have to use transformer..,but i am unable to understand how does transformer actually matches the impedance???

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but how it matches the impedance???
as u have mentioned that there is very high impedance & high o/p voltage requires impedance match. so now suppose u take a transformer & primary of it is connected to the collector of ce configuartion & then speaker is connected to the secondary, WHATEVER MAY BE THE IMPEDANCE OF THE SECONDARY THERE WILL BE AN """"ADDITIONAL IMPEDANCE"""" AT THE PRIMARY ALONG WITH HIGH O/P IMPEDANCE OF CE CONFIGURATION(this is due to well known phenomena i.e impedance reflection), so how do justify about matching the impedance where in the impedances of the primary is going on increasing...??

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so u mean at high frequencies we use transformers & try to match the impedances as u told in cables it's required, but i am unable to understand how transformer matches the impedances???

 

[Audioguru]

u have mentioned that inorder to match the high emitter o/p to the low impedance of the loud speaker(LS) v have to use transformer..,but i am unable to understand how does transformer actually matches the impedance???

No.
A common emitter transistor has the emitter grounded (because it is common to the input and to the output) and the high impedance collectors are the output to the transformer as shown in the amplifiers with an output transformer.

The Philips amplifier has a transformer with a 1200 ohm primary that is fed from the collectors and has a 5 ohms secondary that feeds the speaker. Therefore the transformer is made to match 1200 ohms to 5 ohms.
The amplifier has a 12V power supply which results in a low current (12V/1200 ohms= 10mA peak) in the 1200 ohms primary of the transformer. Therefore the output power of the amplifier is only (7.1mA RMS x 7.1V RMS)= 0.05 Watts. An amplifier that uses low impedance common collector output transistors that directly feed a high current into the speaker produces an output power of about (0.71A x 3.5V RMS)= 2.5 Watts into 5 ohms which is 50 times more power than the amplifier with the output transformer.

but how it matches the impedance???

Ask your teacher or read about it in Google.

as u have mentioned that there is very high impedance & high o/p voltage requires impedance match. so now suppose u take a transformer & primary of it is connected to the collector of ce configuartion & then speaker is connected to the secondary, WHATEVER MAY BE THE IMPEDANCE OF THE SECONDARY THERE WILL BE AN """"ADDITIONAL IMPEDANCE"""" AT THE PRIMARY ALONG WITH HIGH O/P IMPEDANCE OF CE CONFIGURATION(this is due to well known phenomena i.e impedance reflection), so how do justify about matching the impedance where in the impedances of the primary is going on increasing...??

The transformer is designed and made to match the high impedance primary to the low impedance secondary which is the same as matching the low impedance speaker to the high impedance collectors.

so u mean at high frequencies we use transformers & try to match the impedances as u told in cables it's required, but i am unable to understand how transformer matches the impedances???

Cable TV cables are very long and have a fairly high capacitance. Therefore their impedance is only 75 ohms. But their amplifier might have a high output impedance then a matching transformer is used.

 

[crutschow]

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so u mean at high frequencies we use transformers & try to match the impedances as u told in cables it's required, but i am unable to understand how transformer matches the impedances???

The apparent impedance is changed by the turns ratio of a transformer. Fundamentally the impedance reflected to the secondary of a transformer is equal to the primary impedance times the square of the secondary divided by the primary turns ratio [Zs = Zp *(Ns/Np)²]. If you calculate the voltages and currents of the primary and secondary of a transformer you will see how that works.

 

[chuckey]

Lets put 1 W into a 1:2 transformer (twice the number of turn on the secondary compared to that of the primary). So if the input is Vp volts and the number of turns on the primary = T, then every turn has Vp/T volts across it, so by transformer action (twice the number of turns) volts/turn Vp/T times the number of turns on the secondary 2T, gives an output voltage Vs of Vp /T X 2T = 2 Vp.
So the transformer gives out twice the input voltage. If the one watt is Vp X i this must equal Vs X iout, but Vs = 2 Vp, so iout must be 1/2 i, so at the output we have twice the voltage and half of the input current, so by ohms law we have 2 X Vs/ .5 Xi or 4 X Vs/i which is the input impedance and is equal to the turns ratio squared.
Frank