Preamplifier and power amplifier

Hello,
I want to know the difference between pre amplifiers and power amplifiers.
Please answer for it, all electronics beginners will be benefited.

Thanks in advance.

Preamps has very high input impedance, extremely noise figure & a very high gain used to amplify tiny signal picked from sensors before sending it to further amplify by power amps.

Power amps has somewhat lower noise figures, but they can drive bigger loads like loud speakers.

Hope this helps

i will try to simulate a pre amplifier and post it soon.

A preamplifier (preamp),is an electronic amplifier which precedes another amplifier to prepare an electronic signal for further amplification or processing. In general, the function of a preamp is to amplify a low-level signal to line-level, control its volume, perhaps change its input impedance characteristics. A list of common low-level signal sources would include a pickup, microphone, turntable or other transducer. Equalization and tone control may also be applied.
The preamplifier does not, however, provide current. Rather, the second amplifier, which is referred to as the power amplifier, supplies the necessary current to the speakers.

I am attaching a design which is a preamplifier for a low impedance microphone (20Ω-100Ω). Say 50Ω in the present case. The input impedance of the transistor amplifier does not have to be that high to pick up most of the signal. In general make sure that input impedance of preamplifier must be at least 10KΩ more than the impedance of the microphone.

With a 50 ohms source impedance then an amplifier's input impedance of 500 ohms to 1k ohms will be fine. With a 10k input impedance and higher then the amplifier will be noisy without an input.

The collector resistor is 3k ohms. A load resistance of 15k to 30k will be fine. Hardly any power amplifier has a 1M input impedance.

Sorry for my late.
I Thank to every body replied to my questions.Once again i thank lot.
i have one more questions.

Why still we prefer 50ohms load and 75 ohms loads and cables?

Why still we prefer 50ohms load and 75 ohms loads and cables?

Click to expand...

Not generally, only in RF and wide-band applications, where impedance-matching is required. You can refer to the question,
why coaxial cables are mostly designed with 50 or 75 ohms impedance. It has been answered quite often at edaboard.

Hello,

Transmission via cable with specified characteristic impedance serves several goals.

1. RF Power amplifiers want to see a specified load to operate properly
2. Low noise amplifiers also want to see a specified source impedance to give optimum noise performance.
3. High speed signals can be affected by reflection when the back and forth traveling time is no longer small w.r.t signal duration (1/Baudrate).

When wires between a source and a load are no long short with respect to Wavelength, strange things start to appear due to reflections.

A piece of coaxial cable that is electrically 0.25 lambda long appears as an almost short circuit when the end is left open (refer to quarter wave transformer).

When you have a load resistor of 50 Ohms and connect this via a 75 Ohms cable to a source, this source may experience every impedance between 50 and 112 Ohms.

To avoid reflection (think of analog video), the 75 Ohms cable has to be terminated at least on one side. Mostly 2 sides are terminated as even small reflection may lead to shadow images.

Reflection in digital systems result in signal distortion that reduces the noise margin. The actual value is not of importance, but certain realization techniques are limited to certain values.

A power amplifier that wants to see a 50 Ohms load, mostly doesn't have an output impedance of 50 Ohms. Compare this with your audio amplifier at home, It wants to see (for example) 4 or 8 ohms, but its output impedance is generally far below these values.

Besides 50 and 75 Ohms coaxial cable, you also see 100..120 Ohms (twisted wire pairs), 92 Ohms coaxial 62 Ohms coaxial, 300 Ohms symmetrical, 600 Ohm symmetrical (HF feeder to the antenna. Probe leads for an oscilloscope have the highest impedance (very thin inner conductor) to reduce capacitance.