Description:
A low noise pre-amplifier suitable for amplifying dynamic microphones with 200 to 600 ohm output impedance.
This is a 3 stage discrete amplifier with gain control. Alternative transistors such as BC109C, BC548, BC549, BC549C may be used with little change in performance. The first stage built around Q1 operates in common base configuration. This is unusable in audio stages, but in this case, it allows Q1 to operate at low noise levels and improves overall signal to noise ratio. Q2 and Q3 form a direct coupled amplifier, similar to my earlier mic preamp.
Input and Output Impedance:
As the signal from a dynamic microphone is low typically much less than 10mV, then there is little to be gained by setting the collector voltage voltage of Q1 to half the supply voltage. In power amplifiers, biasing to half the supply voltage allows for maximum voltage swing, and highest overload margin, but where input levels are low, any value in the linear part of the operating characteristics will suffice. Here Q1 operates with a collector voltage of 2.4V and a low collector current of around 200uA. This low collector current ensures low noise performance and also raises the input impedance of the stage to around 400 ohms. This is a good match for any dynamic microphone having an impedance's between 200 and 600 ohms. The output impedance at Q3 is low, the graph of input and output impedance versus frequency is shown below:
Gain and Frequency Response:
The overall gain of this pre-amplifier is around +39dB or about 90 times. The first stage Q1 has a gain of roughly Rc / Re or 4.7. This is however reduced by feeding into the input impedance of Q2 and Q3, and the
is an emitter follower and has unity gain. The gain of Q2 and Q3 is roughly 10 / 0.47 = 21 and overall gain therefore 4.7 x 21 = 98. The gain of the circuit may be reduced to 0 by the 47k preset. The response is flat to 100kHz, low frequency rolloff at 30Hz, a simulated plot is shown below:
Output Noise and Overall S/N Ratio:
Any amplifier will add its own noise to the signal, degrading the overall performance. The noise of this preamp measured with a 10k load resistor is shown below.
The Signal to Noise ratio is shown below. Note that the input was simulated as a 200 ohm source at 1 mV amplitude.
Bias Conditions and Operating Point:
The first stage, Q1 was designed to operate with a collector current of 200 uA. With 15k and 47k bias resistors and a 12 V supply voltage the base voltage will be 12 x ( 15 / (15+47)) = 2.9 Volts. The emitter voltage will be the base voltage -0.7 V or 2.2 V. For a 200 uA collector current the emitter resistor will be 2.2 / 0.2 mA =11k. A 10k resistor was used. As Ic and Ie are approximately equal then the collector
The last stage is a composite amplifier similar to my ECM preamp. Q2 operates in common emitter and provides the voltage gain while Q3 operates in emitter follower, buffering the output and has a low impedance output, suitable for driving long cables, if required. The last stage, Q3 was designed for maximum voltage swing, hence Q3 emitter voltage should be around 6V. Variation in transistor parameters however, means that measured voltages will be different to calculated voltages.
Q2 collector voltage is a base-emitter voltage drop higher or 6.7 V and collector current is set at (12 - 6.7) / 10k = 530 uA. This is also the emitter current for Q2 and hence emitter voltage will be ( 0.53 * 0.47) = 0.24 V. The base voltage will be higher by 0.7 V. As the base of Q2 is connected to the emitter of Q3, then the biasing is stabilized to a certain degree against changes in temperature and current gain variation. However if a meter is available for testing transistor beta, then the transistor with the highest current gain should be used for Q2.
P59. ECM Mic Preamplifier
Description:
A microphone amplifier that may be used with either Electret Condenser Microphone (ECM) inserts or dynamic inserts, made with discrete components.
Notes:
Both transistors should be low noise types. In the original circuit, I used BC650C which is an ultra low noise device. These transistors are now hard to find but BC549C or BC109C are a good replacement. The circuit is self stabilizing and will set its quiescent point at roughly half the supply voltage at the emitter of Q2. This allows maximum output voltage swing and also the highest dynamic range.
The electret condenser microphone (ECM) contains a very sensitive microphone element and an internal FET preamp, a power supply in the range 2 to 10 volts DC is therefore necessary. Suitable ECM's may be obtained from Maplin Electronics. Although the schematic is drawn showing a three terminal ECM, two terminal ECM's may be used, the following page in the practical section shows the changes.
The 1k resistor limits the current to the mic. This resistor should be increased to 2k2 if a supply voltage above 12 Volts DC is used and is not needed if the Mic insert is dynamic. The first stage amplifier built around Q1 is run at a very low collector current. This factor contributes to a very high overall signal to noise ratio and low overall noise output. The emitter resistor of Q1 is decoupled by the 100u realizing a maximum gain for this stage. The noise response of the amplifier measured across the 10k load is shown below. Please note that this plot was made with the mic insert replaced by a signal generator.
The second stage, built around Q2 is direct coupled, this minimizes phase shift effects (introduced with capacitive and inductive coupling methods) and acheives a flat output response from 20Hz to over 100kHz. The frequency response measured across a 10k load resistor is plotted below simulated using a 12V power source:
The emitter voltage of Q2 is also fed back to the base of Q1 via resistive coupling. This also ensures bias stabilization againt temperature effects. Q2 operates in emitter follower mode, the voltage gain of this stage is less than unity, however, the overall voltage gain of the preamplifier is about 100x or 20dB as shown in the bode plot above. The output impedance is very low and well suited to driving cables over distances up to 50 meters. Screened cable therefore is not necessary.
This preamplifier has excellent dynamic range and can cope with anything from a whisper to a loud shout, however care should be taken to make sure that the auxiliary equipment i.e. amplifier or tape deck does not overload.
P60. Hi-Fi Preamplifier
Notes
This circuit was submitted by Graham Maynard from Newtownabbey, Northern Ireland. It has an exceptionally fast high frequency response,as demonstrated by applying an 100kHz squarewave to the input. All graphs were produced using Tina Pro.
Squarewave Response with 100kHz Input Signal Applied