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Hi Npatel,
Welcome to our forum.
Amplifiers are designed for a certain output voltage (that you told us) and a certain output current. I could design a simple amplifier for you with only 1mA of output current, then find out later that it must provide 1000 times more. A completely different amplifier would be required but both amplify the voltage the same amount.
Since you also don't say how much input current the amplifier is allowed to draw from the source, I could design a simple amplifier for you that might overload the source, then find out later that it must have a much higher input resistance.

Also, the amplifier won't be simple and will need a compressor to reduce its gain with higher input voltages. You want to amplify 1.2V to 6V, which is a gain of 5 times. But you want to amplify 4V to only 9V, which is a gain of only 2.25 times. Is this correct?

Including the compressor, it would take a few transistors to make the amplifier. A single opamp would do it easily.

Please provide us with all the details for the amplifier: output current, input current and whether a compressor is required.

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Thanks for your reply Audioguru,

here are the specs

input current ~1A
input voltage - 2V
output voltage- 7V

i do not need any current amplification. I am trying to amplify the voltage output of a Li-ion battery and feed it into a 5V regulator so that when the external power is disconnected the system is still running. Typically all 5V regulators that i have seen need Vin of 7-9V to maintain line regulation, thus the need for voltage amplification.
Also i cannot use any opamps. compressors are not required. The amplifier should have high i/p impedance and low o/p impedance to prevent loading.

Thanks for your help.

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Hi Npatel,
Thank-you for providing the extremely important details.
You don't need an amplifier. you need a DC to DC voltage step-up circuit.
Have you ever seen an amplifier that works without a power supply?

The DC voltage step-up circuit consists of a power oscillator that drives an inductor or transformer, a power rectifier and filter capacitor. Since its output will be 7V at 1A, it needs 7W from the battery plus about 1.4W for efficiency loss. Therefore the 2V battery must provide 8.4W to the step-up circuit which is 4.2A!

If your circuit needs regulated 5V, why not just use an 8V or 9V battery?

BTW, "low dropout" voltage regulators are available that need an input of only about 0.3V above their regulated output voltage.

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