In this post we will discuss a few interesting audio circuits using Field effect transistors or FETs
Dual-channel Audio Mixer
In Figure below, you can notice a traditional circuit for mixing two af signals or for choosing one or the other. Similar to the bipolar-transistor circuit, it supplies individual inputs and a common output. Separate gain controls (1 MΩ potentiometers R1 and R2) are supplied, and the resistances of these controls can be elevated to 5 MΩ each if a higher input impedance is required.
Through the usage of 2N4868 FETs (Q1 and Q2), the circuit provides maximum voltage gain of 10 per half. This relates to a maximum signal input of 15 mVrms for an output of 1.5 Vrms before output-peak clipping. The current drain is 350 µA at 15 V DC. All resistors are 0.5 watts and electrolytic capacitor C3 is rated at 25 dcwv. A fourth terminal of the 2N4868 is internally coupled to the metal cover of this FET and must be grounded for shielding.
LC-tune Bandpass AF Amplifier
An amplifier that can be configured for peak output at a required audio frequency is vital in bridge balancing, signal selection, telemetering, cw signal peaking, selective signalling and electronic control. In Figure 2.12A,
You can see such a circuit which uses two 2N4340 FETs (Q1 and Q2) and is tuned using inductance and capacitance.
Figure 2.12B displays the circuit’s frequency response.
Firstly, a common-source amplifier with an unbypassed source resistor (R2) sets the stage. This resistor delivers considerable degeneration for stability and low distortion. The second stage is also a common-source amplifier, with individual outputs from its drain (high impedance) and source (low impedance) to quarter different loads. Using potentiometer R3 that is set for maximum gain, the maximum input signal before output-peak clipping is 7 mVrms. The following maximum signal outputs are AF OUTPUT 1, 2 Vrms; AF OUTPUT 2, 0.45 Vrms. The curve Figure 2.12B is founded on a 1 MΩ load for AF OUTPUT 1. The circuit yields 0.6 mA at 15 V DC.
The inductance and capacitance numbers shown in Figure 2.12A are provided for 1 kHz operation. L1 is a U.T.C type VI-C15, 5.4 H adjustable inductor or equivalent. C2, on the other hand, is a 0.005-µF mica capacitor. The inductance can be set to be varied across a thin range using a tuning screw. This sets the frequency precisely to 1 kHz. Other operating frequencies may be gotten by altering the value of the coil inductance (L1) or the capacitance (C2). If needed, both inductance and capacitance can be mixed.
You must always ensure all wirings are kept short and as straightforward as practical to reduce stray coupling and pickup. The metal container of the 2N4340 is internally attached to the gate electrode, so this FET must be fixed clear of contact with other components. The resistors used for this setup are all rated 0.5 watts.
RC-Tuned Bandpass AF Amplifier
Resistance-capacitance tuning of a bandpass af amplifier is a relatively compact configuration than the inductance-capacitance turning presented in the earlier segment. Figure 2.13A displays a 1-kHz amplifier that is RC-tuned while the next Figure 2.13B presents the frequency response.
The typical amplifier is a three-stage unit, founded on 2N4340 FETs (Q1, Q2, Q3). Using unbypassed source resistors (R4, R10 and R13), a substantial amount of current degeneration is supplied for stability and low distortion. The total voltage gain (including 1 MΩ output loading) yields 42 dB, with potentiometer R11 configured for full output, and the odd number of stages revolves the phase accurately for negative feedback in the path through capacitor C6 back to the input.
Tuning is achieved with a parallel-T network (C3, C4, C5, R5, R6, R7). This is a null network (RC notch filter) attached in the feedback loop between output transistor Q3 and input transistor Q1. C6, which is a 1-µF capacitor, delivers dc blocking for this network, and the 100 kΩ resistor R2 restricts the loading of the network. At the 42-dB gain of the amplifier, the negative feedback through the loop is more than enough to cancel the amplifier gain. Still, the parallel-T network eliminates feedback voltage at the network null frequency. As a result, the transmission by the amplifier becomes extremely sharp at the frequency, as pictured in Figure 2.13B. The values given for capacitors C3, C4 and C5 and for resistors R5, R6 and R7 in Figure 2.13A are for 1-kHz tuning. You may set up the parallel-T networks for other needed frequencies if the following conditions are respected:
The pass frequency of the amplifier then is
- f is the pass frequency of the amplifier in Hertz,
- C is the capacitance of C3 in farads,
- R is the resistance of R5 in ohms.
The amplifier utilises 1.65 mA at 15 V DC. All the resistors are rated 0.5 watt. The capacitors and resistors in the parallel-T network must be chosen to be at 1-percent accuracy.
It is of utmost importance that all wirings are kept short, steady and as direct as possible for uninterrupted operation and to reduce the pickup of stray signals. The gate electrode of the 2N4340 is internally connected to the metal enclosure of this FET. Therefore, it must be fixed out of contact with other components.
RC-tuned Band-suppression (Notch) AF Amplifier
An amplifier that can be configured sharply to eliminate an audio frequency is invaluable for suppressing a heterodyne in radiophone reception, eradicating an unwanted cw signal, splitting one frequency from a mixture, and isolating a single hum or noise component from a complex signal. Figure 2.14A depicts the circuit of a resistance-capacitance-turned amplifier of this type.
This circuit is built upon two 2N3823 FETs (Q1, Q2). Figure 2.14B displays the standard frequency response that can be expected of the circuit. You can see how a significant drop in output of one band of frequencies can be considered into the circuit.
The tuning network consists a parallel-T filter (C4, C5, C5, R4, R5, R6) attached between the amplifier stages. The numbers provided for the filter capacitances and resistances in Figure 2.14A are for 1-kHz removal. The same type of filters can be configured for other frequencies, but the following relationships are respected:
The null frequency is given by this formula
- F = null frequency in Hz
- C = capacitance of C4 if farads
- R = resistance of R4 in ohms
In case the filter capacitors and resistors are neatly chosen, and the capacitors are high Q, the notch point (as shown in Figure 2.14B) will be closely approaching to zero. At some gap on each side of the notch frequency (around 0.1f and 10f), the af output will be around 2 Vrms for a maximum af input of 3.75 mVrms before output-peak clipping. At this point, we can assume the potentiometer R8 is configured for maximum gain and that the amplifier is eliminated with a 1 MΩ resistive load.
We recommend keeping all the wiring as short and as rigid as possible to encourage stability and reduce stray pickup and stray coupling. A fourth pigtail of the 2N3823 is linked to the metal container of this FET and must be grounded, for shielding. All fixed resistors are 0.5 watts and electrolytic capacitors C2 and C8 are rated 25 dcwv. The amplifier consumes 0.2 mA at 9 V DC.
From Figure 2.15, you can see the circuit of typical video amplifier utilising a sole 2N3819 FET. The circuit delivers a voltage gain of 5: the maximum signal input before output-peak clipping, at 1 MΩ load, is 0.6 Vrms for 3 Vrms output. For increasing gain, you may cascade the stages.
The frequency response stays below ± 2 dB from 50 Hz to 4 MHz. Using a screw-tuned slug, you can adjust inductor L1 (Miller No. 4508, or equivalent) from 24 to 35 µH and is fixed for trial purposes for flat output throughout the 50-Hz to 4-MHz range with the steady-amplitude signal applied to the SIGNAL INPUT terminals. For this circuit, the overall current drain is 8.6 mA at 15 V DC.
To reduce strays and enhance operation, short and rigid wirings must be implemented in this circuit. The ratings of resistors R1 and R2 are 0.5 watt.
455-kHz I-F Amplifier
The field-effect transistor will oscillate in an amplifier where the LC tanks in the input (gate) and output (drain) circuits are preset to the exact frequency because of the reverse transfer capacitance of the FET. The whole operation will happen unless the circuit is methodically neutralised. This type of circuit is the classic, transformer-coupled intermediate-frequency amplifier. Neutralisation is quite challenging to do since special tapped i-f transformers for single-ended neutralization are not common except in the step-down type needed by the bipolar transistor.
The i-f amplifier depicted in Figure 2.16 prevents this problem by employing a 455-kHz ceramic filter instead of a transformer. This self-resonant filter does not demand tuning and is attached between two amplifier stages instead of the standard coupling capacitor.
The voltage insertion loss is around 1 dB. The amplifier uses two 2N3823 FETs (Q1, Q2) and delivers an overall open-circuit gain of 400: The maximum i-f input is 2.5 mVrms before peak clipping takes place in the 1-Vrms output. The selectivity of the circuit follows very nearly to that of the filter alone. For example, -3 dB at 2-kHz bandwidth to -40 dB at 160-kHz bandwidth. The whole current drain is 0.2 mA at 9 V DC.
Short and steady wiring ensures the circuit encounters minimum i-f losses and experiences constant operation. A fourth pigtail of the 2N3823 is linked to the metal enclosure of this FET and must be grounded for shielding purposes.
Auxiliary Headphone Amplifier
Almost all times magnetic headphones must be linked to a receiver or some other equipment without significantly loading the device. To achieve this, you would need an auxiliary amplifier with high input impedance.
Figure 2.17 presents the circuit of an amplifier with 2 MΩ input resistance, supplying two magnetic headphones occupying 2 kΩ DC resistance. Based on a sole 2N3823 FET, this amplifier delivers a voltage gain of 40:
This relates to a maximum af signal input of 75 mVrms before peak clipping in the 3-Vrms output signal (potentiometer R1 is tuned for peak volume).
The circuit utilised 1.2 mA at 9 V DC. This power can be delivered by a self-contained battery or can be obtained from a power supply of the device that channels the signal to the headphone amplifier. Resistor R2 is 0.5 watts and electrolytic capacitor C2 is rated at 25 dcwv.
Simple Audio AGC Amplifier
The gain of the FET amplifier stage can be simply governed by adjusting the DC gate bias voltage, the gain regulating contrarywise with the voltage. Once the control voltage is obtained from some vital signal point, like the output of a multistage amplifier (via rectification and filtering), and applied to one of many stages, the results would be automatic gain control.
Figure 2.18 depicts a standard, one-stage af amplifier having this principle. The DC control voltage is applicated at the CONTROL-VOLTAGE INPUT terminals and functions as extra gate bias for the 2N4868 FET (Q1). Starting operating bias is delivered by the voltage drop across source-bias resistor R4. The maximum current received from the control-voltage source is 60 micro-amperes.
Once the DC control is zero, the voltage gain of the stage becomes 10, with a maximum accurate signal of 1 Vrms. At the time the control voltage is 6 V DC, the stage output is lessened to 0.5 mVrms (better than 90 dB range). A lesser DC control voltage can be utilised provided less decibel change is required. For minimum distortion, the maximum af input signal approaching the gate of 2N4868 must be kept to 0.1 Vrms. The whole current drain is 0.5 mA at 9 V DC. The resistors are configured 0.5 watts while the 50-µF source bypass capacitor, C2, is rated at 25 dcwv electrolytic. Also included are the 0.1-µF input and output coupling capacitors. C1 and C3 are 100-V plastic or paper units. A fourth pigtail of the 2N4868 is linked to the metal container of this FET and must be grounded as shown, for shielding reasons.
The circuit is useful in many ways other than standard automatic gain control in an audio amplifier. Whatever application is feasible in which a regulatable or varying DC voltage (at zero power) is available for differentiating the output of an audio channel. Once the control voltage is positive with respect to ground, a p-channel FET (like the 2N2608) must be used on top of adjusting the values of R3 and R4 as needed and reversing capacitor C2.
Simple FET DC Amplifier
Figure 2.19 presents the circuit of a typical one-stage DC voltage amplifier comprising a 2N2608 FET (Q1). This stage occupies no load voltage gain of 5.65.
Stages like these can be cascaded for higher gain but proper setups must be ensured for biasing the gate-source junction at each stage. Once the DC input signal reaches zero at the gate of the 2N62608, there is a maximum drain current approximately 1 mA via drain resistor R2.
This generates a voltage drop across R2, which dampens the DC output voltage to about 0.35 V (false zero). If you need an exactly zero output, the residual voltage can be removed with a standard output-bucking circuit. Once the DC input signal achieves 1.5 V at the 2N2608 gate, the drain current is cut off.
Then, the output climbs to -8.5 V. The output numbers given are for zero loading. This state is gained only when the amplifier channels a high-resistance device like an oscilloscope, vtvm or any other amplifier. (At 10 kΩ load, the output drops to 5 V DC).
Since a positive input signal provides a negative output signal, the amplifier is an inverter as well. This feature was well received. The unit also is a type of current amplifier where all of the signal-input current is considered in gain-control potentiometer R1 (1.5 µA at 1.5 V input). The output current, on the other hand, flows via an external load due to the amplified DC voltage. This is 500 µA in a 10 kΩ load with a current gain of 332. (The current and voltage gains discussed here present a DC power gain of 1000000 plus).
A typical DC amplifier of this kind is valuable in control systems and instruments. For example, it will drive a 10-kΩ, 5-V DC relay, equipped with an input signal of only 1.5 V at 1.5 µA. many thresholds of operation can be gotten easily by regulating the gain control R1. The layout of the amplifier is not so important. But, the 2N2608 must be attached away from wiring and other electronic components, as the gate lead is internally attached to the metal container of this FET. The rating of the drain resistor R2 is 0.5 watts.