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# Voltage amplifier, Av=5400. Solutions ?

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#### Hellen Grig

##### New Member
I have to design a sinusiodal voltage amplifier with a voltage gain of 5400 and a specified bandwitdh . Wich soluttion is better :with transistors or with OA?

What is the bandwidth?
What is the load? (current, voltage output)

What is the bandwidth?
What is the load? (current, voltage output)
The only conditions I have are:
Vin=5mV
Vout=27V
f=[270Hz, 7KHz]
Rout= 16ohms

Use a microphone preamp (transistors or opamp) with an audio power amp IC driving a transformer.
But a suitable audio transformer is probably not made anymore so you will need to design a transistor amplifier to produce the 27V (77V p-p) output.
You will need a power supply with about 82VDC output.

Use a microphone preamp (transistors or opamp) with an audio power amp IC driving a transformer.
But a suitable audio transformer is probably not made anymore so you will need to design a transistor amplifier to produce the 27V (77V p-p) output.
You will need a power supply with about 82VDC output.
I already thought to a Common Emitter Amplifier , 2 stages, maybe , but the problem is that I don’t Really know What TBJ to use . What characteristics I m looking for ?

What is a "TBJ"?
The output power of 27V into 16 ohms is 46W. A common emitter class-A stage will get extremely hot all the time even when there is no input signal. You need a complementary class-AB pair of emitter followers plus at least two transistor common emitter amplifiers.

Is Vout 27 volts peak, peak-to-peak, or RMS?

Does the load have to be connected to ground? If not, then two ampl running BTL will cut the power supply voltage requirement in half.

ak

What is a "TBJ"?
The output power of 27V into 16 ohms is 46W. A common emitter class-A stage will get extremely hot all the time even when there is no input signal. You need a complementary class-AB pair of emitter followers plus at least two transistor common emitter amplifiers.
BJT , sorry . TBJ is in my language .))
Is not easier to do this with OA ?

Is Vout 27 volts peak, peak-to-peak, or RMS?

Does the load have to be connected to ground? If not, then two ampl running BTL will cut the power supply voltage requirement in half.

ak
27 V RMS .
The load must be Connected to Ground , I think .

i would go with two sections. the first would be an op amp with Av set at 100 or 200. the second section would be an LM3886 with Av=54 (if the op amp Av=100) or Av=27 (if the op amp Av=200)

you probably could get away with using just an LM3886 with an Av=5400, but if it were me i would use the two stages.

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27 V RMS . The load must be Connected to Ground , I think .
Then you're looking at power supplies of +/-39 Vdc for the output waveform, plus some headroom for the output transistors, so somewhere around +/-45 Vdc at a minimum of 2.4 Apeak.

When you comb through the specs, the LM3886 does not meet the output voltage requirements. It comes within about 2 V.

ak

I speak in words, not letters. A Bipolar Junction Transistor is clear in any language. An Opamp is also very clear but OA could be anything.
The output current of almost all opamps is way too low for a 16 ohms load.

I speak in words, not letters. A Bipolar Junction Transistor is clear in any language. An Opamp is also very clear but OA could be anything.
The output current of almost all opamps is way too low for a 16 ohms load.

Our teacher said us that is easier to design it with OpAmps . I used 2 stages , first Av=90 and second stage has Av=60. It works ok , I have 27V from 5mV ,but I ignored load resistance . When I place it, the output voltage is limited. How can I solve this ?

Our teacher said us that is easier to design it with OpAmps . I used 2 stages , first Av=90 and second stage has Av=60. It works ok , I have 27V from 5mV ,but I ignored load resistance . When I place it, the output voltage is limited. How can I solve this ?

As has already been explained, you can't use an opamp for the output, you need a power amp.

You can
a) Use a power opamp that can drive the load directly.
b) Use an audio power amplifier device (similar to the LM3886 mentioned above) that can drive the load directly.
c) Design a discrete amplifier circuit that can drive the load directly.

ak

if you use an op amp as the input stage, you will need a power amplifier stage with voltage gain. most op amps run on supplies of 36V (+/-18V) or less.

if you need to simulate an LM3886, the PSPICE model can be found [here] (this model will work with LTSpice). the page on TI's website calls this a "20-94V" device, meaning it can be used with up to 47V rails so it's likely it will work, at least for the 16 ohm load, using 45V rails.

if you do a search for "power op amp", in addition to the LM3886, you will likely get a lot of hits for devices made by Apex. if you are wondering why you don't see a lot of these amazing devices used in audio equipment, it's because their prices are very high. an LM3886 can be found for about $10 the Apex equivalent is about$200