5 Simple Preamplifier Circuits Explained

As the name suggests a preamplifier circuit pre-amplifies a very small signal to some specified level that can be further amplified by an attached power amplifier circuit. It basically acts like a buffer stage between the input small signal source and a power amplifier. A preamplifier is used in applications where the input signal is too small and a power amplifier is unable to detect this small signal without a preamplifier stage.

The post explains 5 preamplifier circuits which can be quickly made using a couple of transistors (BJTs) and a few resistors. The first idea is based on the request presented by Mr. Raveesh.

Circuit Objectives and Requirements

1. Electronics is my hobby since so many years. Often I will be browsing your website and found many useful projects. I require a favor from you.
2. I have a FM transmitter module which works on 5 volts DC with provision to connect from Computer through USB or from audio out from any other device through 3.5 mm audio jack.
3. The module works great in computer USB mode with great signal strength, quality and coverage. But when I connect the same through audio input jack from DTH set top box the signal strength becomes weak even with full volume in both set top box and FM module. I think the audio signal level from set top box is not sufficient for the FM module.
4. Please suggest me a good quality stereo audio small signal preamplifier circuit which can work from 5 or 6 volts single supply, that would not load the set top box, preferably using good low noise op-amp with detailed circuit and parts label.

1) Preamplifier using two Transistors

A simple pre-amplifier circuit can be very easily built by assembling a couple of transistors and some resistors as shown in the following figure:

The circuit is a simple two transistor pre-amplifier using a feedback loop for enhancing the amplification.

Any music as we know is in the form of a consistently varying frequency, therefore when such a varying input is applied across the indicated C1 end terminals, the same is delivered across the base T1 and ground.

The higher amplitudes are processed normally and is reproduced with a potential that's approximately equal to the supply voltage, however for the lower misc amplitudes T2 is allowed to conduct at the higher ratio which is allowed to pass to its emitter.

At this time when the actual enhancement of the music is implemented by transferring this accumulated higher potential back to the base of T1 which correspondingly saturates at a much optimal rate.

This push pull action ultimately results in an overall amplification of an insignificantly small music or data input into a significantly larger output.

This simple circuit enables boosting extremely small or minimal frequencies to an appreciably bigger outputs which can be then used for feeding lager amplifiers.

The discussed circuit was actually popularly used in old cassette type playback recorders in their preamp stages for boosting the minute signals from the tape head so that the output from this small amplifier became compatible for the attached high power amplifier.

Parts List

• R1 = 22K
• R2 = 220 ohms
• R3 =100k
• R4 = 4K7
• R5 = 1K
• C1 = 1uF/25V
• C2 = 10uF/25V
• T1/T2 = BC547

Adjustable Preamplifier Circuit

This useful preamplifier circuit is an enhanced version of the above design. It has a voltage gain which can be set at any level between five and one hundred times by using a feedback resistor of the appropriate value.

The input impedance is high, being typically about 800K and a low output impedance of around 120 ohms is obtained.

The noise and distortion produced by the circuit are both very low.

A maximum output signal level of about 6 volts peak to peak can be handled before clipping occurs.

The figure shows the circuit of the unit, and this is a straight Forward two transistor, direct coupled arrangement, with both transistors being used in the common emitter mode. R2 provides local negative feedback over Tr1, and provides a convenient point tn which overall negative feedback can be applied to the circuit.

This feedback is obtained from the collector of Tr2 via D.C. blocking capacitor C3 . and the value of RF determines the amount of feedback that is applied to the amplifier. The lower the value of this component the more feedback that is applied, and the lower the closed loop voltage gain of the unit.

The required value of Rf is found by multiplying the required voltage gain by 560. Thus, a voltage gain of ten limes, for example, requires Rf to have a value of 5.6k.

It is recommended that the voltage gain should be kept within the limits stated earlier. C2 rolls of the high frequency response of the amplifier, and is necessary as instability might otherwise occur.

The upper -3dB response of the unit is still at about 200kHz even if the amplifier is used at a voltage gain of hundred times. When used as lower gains the upper -3dB point is pushed proportionately higher. The lower -3dB point is at approximately 20 Hertz incidentally.

Another Transistorized Preamp design

This is a high impedance input 2 stage preamplifier that features an adjustable voltage gain, from 1.5 to 10. This gain can be varied by setting up VRI and becomes handy where the MIC sensitivity required to be varied often.

As shown above, the circuit is actually designed for crystal microphones or ceramic cartridges.

Parts List

2) Using an FET

The second preamplifier design looks even simpler as it works using a single low cost JFET. The circuit diagram can be seen below.
The circuit is self explanatory, and can be integrated with any standard power amp for further amplification.

Guitar Preamplifier

It usually becomes necessary to hook up an electric guitar with a mixing panel, a audio deck or a portable studio.

As much as wiring is concerned, that may be not an issue, however matching the high impedance of the guitar component with the low impedance of the line input of the mixing panel does becomes an issue.

Even the unsuspecting high impedance inputs of those units aren't well suited for the guitar output. As soon as the guitar is plugged into this kind of input, you hardly see a signal feasible for the panel or deck to process.

It might be likely to attach the guitar to the (high-impedance) mic input, however that is commonly way too sensitive for the function, which leads to clipping of the guitar signal too easily.

The matching amplifier introduced in this article answers these difficulties: it features a high- impedance (1M) input that will stand up to voltages of over 200 V. The output impedance is fairly small. Amplication is X2 (6 dB).

Dual tone control, presence control and volume control are offered. The circuit is designed for input levels of up to 3 V. Over this level distortion rises, but that may be, naturally, a decent outcome having guitar music.

True clipping of the input signal is not going to take place until eventually significantly bigger levels above the minimum guitar specs are utilized. The circuit is powered by a 9-V (PP3) battery through which the circuit pulls a current just around 3 mA.

3) Stereo Preamplifier Using IC LM382

Here;s another nice little preamp circuit using a dual opamp IC LM382. Since the IC provides a dual opamp package two preamps could be created for stereo application. The output from this preamp can be expected to be very good.

Parts List

R1, R2 = see the below given table.
R3, R4 = 100K 1/2 watt 5%
C1, C2 =100nF polyester
C3 to C10 = see table
C11 to C13 = 10uF/25V
IC1 = LM382

4) Balanced Preamp

If you are looking for something more sophisticated, you may want to try this balanced preamplifier design. The circuit is elaborately explained in this article which you can refer for your reading pleasure.

5) Preamplifier with Tone Control

A tone control normally includes bass and treble features for tweaking the dynamic quality of the music.

However, since a tone control also has the ability to amplify the incoming it can be effectively used like an outstanding Hi-Fi preamplfier circuit stage.

This we have a system which works two ways, foe enhancing the tone quality of the music and also preamplifying the music for the subsequent power amplifier stage.

The complete circuit of this fifth preamplifier can be seen below:

UPDATE

Here are a couple of more preamplifier circuits that may interest you.

6) Low Z (impedance) MIC Preamplifier Circuit

The circuit described so far, are of course, only suitable for use with high impedance microphones, and provides insufficient gain for use with low impedance types.

These usually provide an output signal level of about 0.2mV. R.M.S., which is about one tenth of that generated by a high impedance microphone.

The circuit diagram is for a preamplifier that can be employed with low impedance microphones, and should give an output signal of around 500mV. R.M.S.

The prototype was found to work well with both 200 ohm and 600 ohm impedance dynamic microphones, but it should also work well with electret types which have a built-in FET buffer amplifier, but no step-up transformer.

The unweighted noise performance of this circuit is not quite as good as that of the previous circuit, but is still about -60dB referred to 500mV R.M.S.

This circuit is really an adaptation of the second design. The FET input stage uses the common gate mode rather than the common source one.

The common gate configuration gives reasonably good voltage gain together with a low input impedance (a few hundred ohms) which matches the microphone reasonably well.

The only other change in the circuit is that the emitter of Tr2 connects direct to the negative supply rail and there is no feedback resistor here.

This is done to boost the gain of the circuit, which as explained earlier, needs to be about ten times higher for a low impedance microphone.

Zero Noise Preamplifier Circuit

In numerous applications (audio, computing devices, aerospace amplifiers, communications, etc.) an exceptionally low-noise preamplifier stage becomes necessary, and just about any model strategy that could minimize noise by even 1 dB is welcomed with passion by everybody involved.

The circuit demonstrated below provides a fundamental design concept, although not quite ideal, the final results to date are encouraging.

Applying even the highly sensitive measuring devices at our fingertips we still could not determine virtually any output noise signal whatsoever!

Having said that, currently there seems to be still one leftover issue: the gain of the circuit is zero.

Automatic Gain Control Preamplifier Circuit

This microphone preamplifier features automatic gain control, which maintains the output quality relatively consistent within a broad selection of input ranges.

The circuit is particularly well suited for driving the radio transmitter modulator and enables a large typical modulation index to be accomplished.

This can possibly be applied in power amp systems and intercoms to deliver better intelligibility and make up for varied speakers specifications.

The specific signal amplifier stage is T2, which works in common emitter mode, the output signal being extracted from its collector.

A part of the output signal is supplied by means of emitter follower T3 towards a peak rectifier containing D1/D2 and C4. The voltage across C4 is employed to regulate the T1 base current, that constitutes section of the input attenuator.

At reduced signal concentrations the voltage on C4 is minimal, and T1 pulls very little current.

When the input signal level rises, the voltage on C4 goes up and T1 switches on harder, causing higher suppression of the input signal.

The overall effect is that as the input signal heightens it has to go through an increased degree of attenuation and the output signal thus continues to be reasonably constant across a wide range of input signals.

The circuit is appropriate for inputs having a peak input level up to 1 volt. The microphone could be substituted by a tiny loudspeaker for converting the circuit into an intercom.

1.5 V Preamplifier circuit

While most amplifiers come without adequate input sensitivity and hardly any room within their enclosure, independent low power pre-amplifiers which could be integrated externally may be very useful.

These need to have a bare minimum number of parts and likely be powered with just one dry cell.

The independent 1.5 V preamplifier circuit explained below is made up of individual amplifying transistor preceding an emitter follower. DC negative feedback keeps the operating level stable.

The gain is roughly x 10 to x 20. If the signal source provides impedance of more than 100 k ohms, some amount of gain control is possible through P1.

A reasonably long-term battery back up could be acquired through the use of a couple of 1.5 volt dry cells (in series) rather than one.

If the power falls under 1 volt the amplifier may stop functioning.

Typical dry cells frequently deplete quickly to 1 volt and have to subsequently be thrown away, although it may take longer for each one of two cells to drop to 0.5 volt. Current draw at 3 volt supply will probably be around 450 microamps.

Miscellaneous Preamp Circuits

Micro-volt Preamplifier

This preamplifier design can amplify extremely small input signals in the range of microvolts

As a result, the preamplifier must give a significant amount of voltage gain in order to match the output to a hi-fi amplifier, which requires a signal level 1,000 times greater.

Because the input signal may climb at a rate of 6dB per octave, the preamplifier must also offer equalization.

At increasing audio frequencies, nevertheless, low microvolt frequencies are inefficient and require considerably lower roll-off. Q1 and Q2 are utilized in a typical two-stage, direct coupled, common emitter amplifier, with frequency-selective negative feedback provided by C3 and R4.

In addition, the midband voltage gain of the input stage is adjusted at around 46dB. With that kind of a low input level, it is apparent that low noise transistors (such as the BC109C) are required to get excellent performance.

It additionally helps to operate Q1 with a low collector current, approximately 200uA. Q3 serves as a low gain common emitter stage, providing extra amplification. R9 adds negative feedback to adjust the voltage gain of Q3, and the given value yields a gain of around 14dB.

Enhanced MIC Preamplifier Circuit for tailored Output Response

In the realm of speech communication, customizing the response of the preamplifier that follows the microphone enhances speech clarity and minimizes unwanted background noise interference affecting the signal.

This circuit is engineered to achieve the necessary roll-off slopes below 300 Hz and above 2 kHz. Notably, the higher frequencies experience a considerably swifter attenuation compared to the lower frequencies.

The input stage utilizes a JFET to establish a high-impedance input, which is then directly linked to Q2, a common-emitter stage with bootstrapped feedback to Q1.

The circuit operates with a 12 V power supply and can accommodate both low and high-impedance dynamic microphones.

In instances where a low-impedance microphone is employed, it may necessitate the addition of an appropriate load resistor across the input connector. RV1 is responsible for regulating the output level.