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Frequency response of bjt amplifiers for microphones

We know that the an amplifier can produce enlarged but exact version of small input signal at the output of an electronic circuit. Electronic devices like TV, radio, tape recorder etc. These devices do not operate at the same way. Some devices need to amplify the current, some devices require to amplify voltage etc. Again, in some devices a part of input wave is to be amplified.


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WATCH RELATED VIDEO: FREQUENCY RESPONSE OF BJT AMPLIFIER - UNTUNED AMPLIFIER -- ELECTRONIC CIRCUITS

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Transistors are the key components in many different kinds of audio preamplifiers, amplifiers, and tone-control circuits. Recent articles in this series have discussed the operation principles and applications for discrete bipolar junction transistors BJT. Earlier articles have covered such subjects as low-power amplifier circuits, multivibrators, and oscillators.

Audio Amplifier Basics: A modern stereo amplifier system has two closely matched high-fidelity audio amplifier channels. Typically each of those channels offers switch-selectable inputs for such signal sources as a tuner, tape-player, CD-player, TV, MTS, etc. Each also provides a single output signal to a high-power loudspeaker. To analyze one of those systems, it is useful to divide the system into three functional circuit blocks, as shown in Fig.

This block might also contain additional filter circuits including one specifically designed to screen out scratch and rumble. The last section of the amplifier system is the power amplifier. It might be able to produce power levels from a few hundred milliwatts to hundreds of watts. Audio power amplifiers are designed to cover the audio frequency range with minimal distortion.

Most quality products today include automatic overload and thermal-runaway protection. The three sections of the audio amplifier system are all powered from a single built-in power supply. All three sections include individual power supply decoupling networks to prevent unwanted signal interference. The first two amplifier blocks will be discussed here. However, if the input is obtained from a micro-phone or other audio input device, it will probably need preamplifier conditioning.

Magnetic transducers typically offer low output impedance and a low signal sensitivity of about 2 millivolts. Their outputs must be fed to a high-impedance preamplifier stage with near-unity voltage gain. Most microphones have a near flat frequency response, so they can be matched to simple, flat-response preamplifier stages. Figure 2 shows a unity-gain preamplifier circuit that will work with most high-impedance ceramic or crystal microphones. It is an emitter-follower common-collector amplifier with an input network bootstrapped by C2 and R3.

It has a typical input impedance of about 2 megohms. The combination of C5 and R5 decouples the amplifier from the DC power supply. Figures 3 and 4 show alternative preamplifier circuits that will match magnetic microphones. The single-stage circuit of Fig. The two-stage circuit of Fig. RIAA Preamplifier Circuits: The replay of a constant-amplitude 20Hz to 20KHz variable-frequency signal that has been recorded on a phonograph disc with conventional stereo recording equipment will generate the nonlinear frequency response curve shown in Fig.

Here, the dotted line shows the idealized shape of this curve, and the solid line shows an actual shape. Examination of the idealized dotted version of the curve in Fig.

The response then flattens at frequencies below 50Hz. There are good—but difficult to explain—reasons why the precise Fig. However, all you really need to know is that they make it possible to produce disc recordings with excellent signal-to-noise ratios and wide dynamic ranges.

The curves were applied during record pressing. The important point to be made here is that when a disc is replayed, the output of the pickup device must be passed to the power amplifier through a preamplifier whose frequency equalization curve is the mirror image exact inverse of the one used to make the original recording.

As a result, a linear overall record-to-replay response is obtained. Figure 6 shows the RIAA equalization curve.

RIAA is an abbreviation for the R ecording I ndustry A ssociation of A merica, the organization that standardized the precise specification of the curve for the equalization of phonograph records. When long-playing phonograph record-player records were the primary source of recorded music and audio entertainment, circuit designers had to include filter networks that corrected the input from the record to conform to the RIAA equalization curve.

The relatively recent world-wide conversion to compact discs CDs as the primary source of recorded music and entertainment has diminished the importance of the RIAA curve. Equalization is not required for linear signal sources such as CDs. Nevertheless, a preamplifier with an RIAA equalization network is still needed if you want to play any of the pressed long-playing and 45 rpm records.

This equalization can be obtained by wiring frequency-dependent, resistive capacitive feedback networks into a preamplifier. This circuitry causes the gain to fall as the frequency rises.

One network will control the 50 to Hz response, and the other will control the Hz to 20 kHz response. Figure 7 is the schematic for an amplifier with those networks that will work with any magnetic phono cartridge. The preamplifier circuit is designed around transistors Q1 and Q2, with C2 and R5, and C3 and R6 forming the feedback resistor capacitor equalization network.

The output of the emitter-follower buffer stage, transistor Q3, can be controlled by volume control potentiometer R The quality of reproduction of ceramic or crystal phono cartridges is generally lower than that of magnetic cartridges, but they produce far higher amplitude output signals.

Ceramic and crystal phone cartridges will work with simple equalization preamplifiers—one reason why those cartridges were installed in so many low-cost record players. Both circuits are designed around transistorized emitter-follower output stages Q1 and Q2. The output of the circuit in Fig. Two-stage equalization is provided by the resistance-capacitance network made up of C1, C2, R2, and R3.

The other part of the network is formed by C1 and R3. At 50 Hz, this circuit has a high input impedance of about kilohms, which causes only slight cartridge loading. However, as frequency increases, input impedance decreases sharply, increasing cartridge loading and effectively reducing circuit gain.

The equalization curve approximates the RIAA standard, and circuit performance is adequate for most practical applications. A Universal Preamplifier: Most audio amplifier systems must have preamplifiers with many different characteristics. These include high-gain linear response for magnetic microphones, low-gain linear response for tuners, and high-gain RIAA equalization for magnetic phone cartridges.

To meet this broad requirement, most amplifier designers include a single universal preamplifier circuit such as the one shown in Fig. Basically a high-gain linear amplifier, its characteristics can be altered by switching alternative resistor filter networks into its feedback system.

For example, when the selector switch is set to the Mag phono position, alternative input sources can be selected by S1-a, and appropriate linear-response gain control feedback resistors R8, R9, and R10 are now selected by S1-b. Those feedback resistor values are selected between 10 kilo ohms and 10 megohms to suit individual listener tastes.

Circuit gain will be proportional, to the feedback resistor value. Volume Control: The Volume control circuitry of an audio amplifier system is normally located between the output of the preamplifier stage and the input of the tone-control circuit.

It is usually only a potentiometer within the circuit, as shown in Figs. However, the catch here is that rapid rotation of the potentiometer knob can apply DC voltage to the next circuit for brief intervals. That voltage could upset circuit bias and cause severe signal distortion. The block diagram in Fig. It is fully DC-isolated from the output of the preamplifier by capacitor C1, and from the input of the tone-control circuit by C2.

As a result, variation of the wiper of control potentiometer R1 has no effect on the DC bias levels of either circuit. Potentiometer R1 should have a logarithmic taper, that is, its output should be logarithmic function rather than linear. He can, for example, boost or reduce the low-frequency treble sections of a musical selection to emphasize the sounds of specific sections of the orchestra.

Tone-control networks typically consists of simple resistive-capacitive filters through which the signals are passed. Because these networks are passive, they cause some signal attenuation.

Tone control networks can, if desired, be wired into the the feedback loops of simple transistor amplifiers to give the system an overall signal gain. Those are known as active tone control circuits.

Capacitors C1 and C2 are effectively open circuited when the frequency is at its lowest bass value. It can be seen from Fig. This arrangement results in a low resistive value of about ohms that only slightly attenuates bass signals. The Fig. Finally, in Fig. This circuit resistance value is equal to kilohms divided by 11 kilohms.

It gives a signal attenuation of about 20 dB at all frequencies. As a result, the circuit gives a maximum bass boost of about 20 dB or cut relative to the flat signals. This circuit also provides about 20 dB of signal attenuation when potentiometer R3 is in the flat position, and it gives maximum treble boost or cut values of about 20 dB relative to its flat performance.

Finally, Fig. Active Tone Controls: A tone-control network can be included in the feedback path of a transistor amplifier so that the system will have an overall signal gain rather than attenuation when its controls are in the flat position. These networks can be simplifier versions of the basic circuit shown in Fig. A comparison of Figs. It can be seen that the two capacitors C1 and C2 of Fig. Similarly, the treble version of Fig.

Resistors R3 and R4 balance the performance of the two section of the Fig. This mixer permits several different audio signals to be mixed together to form a single composite output signal. Figure 16 is the schematic for a three-channel audio mixer that will provide an overall gain of one between the output and each input channel. Each input channel includes a single 0.

The number of input channels to this audio mixer can be increased by adding more capacitors and resistors with the same values as C1 and R1. The mixer should be located between the output of the tone-control circuitry and the input to the power amplifier. One input should be taken from the output of the tone-control circuit, and the other inputs should either be grounded or taken from the desired source. Your email address will not be published.


Chapter 6 Transistor (BJT) Amplifiers – Electronic Circuit Analysis

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These microphones are small, have excellent sensitivity, a wide frequency response Which amplifier has the BJT as the most commonly used amplifier? a.

Electret microphone amplifier – Terrible low frequency response


Application publication date : The invention discloses a high-gain front-arranged amplifier for an electret electrochemical machining ECM microphone used in a communication system. A P-channel metal oxide semiconductor PMOS source electrode follower is adopted to be used as an input end, the size of a PMOS M3 does not need to be big, and input capacitance can be controlled in 2pF, so that high gain of the amplifier can be increased. A PdOS source electrode follower is adopted to be used as an input end, an input end of an amplifier A is easily arranged to be direct current DC bias voltage, complementary metal-oxide-semiconductor transistor CMOS integrated circuit manufacturing technology is adopted, and manufacturing cost is reduced. A kind of electret ECM microphone high-gain preamplifier Technical field. The present invention relates to a kind of CMOS integrated circuit, refer specifically to a kind of electret for communication system ECM microphone high-gain preamplifier. Speech communication system mainly still realizes that with electret ECM microphone voice signal is to the transformation of the signal of telecommunication now.

Electronic amplifiers

frequency response of bjt amplifiers for microphones

This is a simple mic amplifier circuit which is using two stages first a mic preamplifier circuit stage and second a audio amplifier circuit stage. The low-noise microphone-bias-voltage generator can bias most electret microphones. A slight innovation eliminated a couple of external The output is usually very low voltage, around 5 to 10mV. The first one is a single transistor simple microphone circuit, very simple to hook up using an electret microphone or MIC and an audio amplifier.

Chapters 5 through 7 covered diodes, light-emitting diodes LEDs , amplifying devices, and operational amplifiers. In particular, we saw on a top level how to use negative feedback with op amps.

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This task is aided through the use of such circuit design and testing programmes. Sound pressure moves one of its plates. The movement of the plate changes the capacitance. The electrets capacitor is connected to an FET amplifier. These microphones are small, have excellent sensitivity, a wide frequency response and a very low cost.

Explain Frequency Response

In the previous episode, we examined the benefits of a better preamp in terms of lower noise. But what could a better preamp do to improve your sound? While technical properties like noise can be measured objectively, sound is a subjective and therefore more elusive concept. Nearly all microphone preamps measure essentially flat within the audible range from 20 Hz to 20 kHz. At least at gain settings up to about 50 dB.

common base, and common collector BJT amplifiers are summarized in Table to The frequency response of the CB amplifier is much larger than the CE.

Génie électrique (ELG)

I am looking at creating a simple high pass filter for a microphone breakout board which will be attached to a MCU. I'm not an electronics engineer so this is probably a simple type question for those who are. I've learnt from Wikipedia that the formula for determining the cut off frequency is:. Now what I am trying to learn is how else does R and C impact the audio signal as this presents me with 2 degrees of freedom.

Classification of BJT Amplifier

RELATED VIDEO: ECD8 - Low frequency response - BJT amplifiers - Roll off

Understanding these impedance values is crucial to master microphones truly. So what is microphone impedance? Microphone signals are AC voltages. Impedance controls the flow of the audio signal. Microphone impedance bridging is critical for the optimization of your microphones. The Ultimate Illustrated Guide!

An amplifier , electronic amplifier or informally amp is an electronic device that can increase the power of a signal a time-varying voltage or current. It is a two-port electronic circuit that uses electric power from a power supply to increase the amplitude of a signal applied to its input terminals, producing a proportionally greater amplitude signal at its output.

They almost always require a preamplifier of some sort to boost the signal to a useable level. Recently, I wanted to use an inexpensive electret microphone for a small short-range public address speaker and I needed to make an amplifier for it. I always follow a logical step-by-step process when I design something. Figure 1: A IC power amplifier that operates a three inch speaker. The first thing I did was to look at the specs for the microphone mike.

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