DE1930676A1 - Preamplifier for sound receivers, especially for capacitive converters - Google Patents

Description

17 152

Akustische u, Kino-Geräte Gesellschaft m «bH, in 1150 Vienna , Nobilegasse 50 (Austria)

Preamplifier for sound receivers, especially for capacitive converters

The invention relates to a preamplifier for sound receivers, in particular for capacitive transducers, which preamplifier is equipped with at least one field effect transistor.

Compared to conventional tube circuits, which have hitherto been used, for example, in condenser microphones, results in the use of field effect transistors a lower circuit complexity, a better Hausch behavior of the preamplifier, as well as lower microphony and a longer service life, combined with a lower susceptibility to faults. The omission of the heating circuit not only avoids the risk of hum interference, it can also build very simple supply voltage circuits because of the relatively low supply voltages.

For example, when using a two-wire shielded Cables and center-tapped transformers, a phantom circuit for supplying the supply voltage can be used for the preamplifier, in which the two audio lines as the forward conductor and the shielding of the cable

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serve as a return conductor. If a three-core cable is available, a third cable can be used instead of the shielding jacket ladder can be used.

In practical studio operation, however, there are difficulties insofar as the equipping of the amplifiers with pnp transistors (positive pole of the supply voltage grounded) is as common as that with npn transistors (Negative pole of the supply voltage grounded), so that damage can occur through incorrect polarity.

It is therefore of great importance for the studio technician to determine the polarity of the supply voltage from the Knowing the individual amplifiers just as well as the one equipped with a transistorized preamplifier Sound receiver, which is an additional burden on the technician. In particular is. but it is disadvantageous that because of the necessary selection options, actually more devices must be available than is necessary.

The invention aims to obviate all of these drawbacks, i.e. a preamplifier to create which has bipolar properties with respect to the polarity of the supply voltage source and can therefore be operated with both a grounded minus cable and a grounded plus cable without switching can.

This is the case with at least one field effect transistor equipped preamplifier for sound receivers, in particular for capacitive transducers, according to the invention achieved essentially in that the load resistances R-, Rp of the stage in the drain and source circuit are equal or approximately the same, the output voltage between. Drain and Source is taken and the The operating point is set in such a way that the potentials of the source and gate are both positive and negative Supply voltage are the same or approximately the same, which means that the transmission properties are the same regardless of polarity

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The invention makes use of the fact made that field effect transistors, seen physically, represent a quasi symmetrical arrangement. One can so interchange the roles of the drain and source electrodes, if it is ensured that between the burr electrode and the appropriate electrode used as the source Preload is available. It is therefore expedient to use, preferably, fully symmetrical field effect transistors in the circuit according to the invention however, field effect transistors that only meet the power conditions or the cut-off frequency can also be used comply if they are used according to the marking, because in the circuit according to the invention no special requirements regarding performance or maintenance at high frequency need to be met.

The idea on which the invention is based exists accordingly, therein, the symmetry properties of a field effect transistor exploit, whereby by special circuit measures one of the polarity of the supply voltage dependent bias voltage for the gate electrode is generated, so that the field effect transistor, no matter with which Polarity the supply voltage is switched on, the correct operating point setting is obtained.

Other features and advantages of the invention can the following description based on the drawing in which in FIGS. 1 and 2 show two exemplary embodiments.

In Fig. 1, the circuit of a preamplifier according to the invention in connection with a condenser microphone M is shown. This receives a polarization voltage Up via the series resistor R _v . The non-grounded electrode of the capacitor microphone K is connected to the electrode labeled 3 of a field effect transistor by means of the coupling capacitor Cjc. This electrode is commonly called the "gate"

909882/1558 _oBielN M.

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■ designated. The two other electrodes of the transistor are connected to the primary winding of an output transformer Tr via coupling capacitors C. A resistor R4. leads from the electrode 2 to a pole of the supply voltage (^ Ui ₀ ), whereas the electrode 1 is via the resistor R _{5 on the reference line} . According to the invention, the two resistors R., R ^ are at least approximately the same size.

A voltage divider consisting of the resistors R ₁ , R, is directly connected to the supply voltage, that is between i Ub and the reference line. A preferably high-resistance resistor R _v leads from its tap to the gate electrode, whereby the latter receives its bias voltage. In parallel with the resistor IU of the voltage divider, there is now an auxiliary circuit consisting of the series connection of the diode D with an ohmic resistor Rg.

It is easy to understand that through this auxiliary circuit, the voltage divider above the supply voltage has become polarity dependent, depending on the polarity the supply voltage this once in proportion

RR RR

R ₅ : (R- ^ R ₅ ) or t (R ₁ + ) shares and with it

shifts the bias voltage of the gate electrode according to the mode of operation of the field effect transistor. In the first case it is assumed that the polarity of the supply voltage is such that the diode D is blocked. The resistor Rg can therefore not act as a parallel resistor to R ₅ . If, however, a current flow through the diode D is possible due to the corresponding polarity of the supply voltage, the resistor R _{2 is directly parallel to the resistor R 5} , neglecting the diode resistor, so that the gate bias has a lower value than the electrode 1. This case occurs when there is a positive voltage at terminal tü ^. The field

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Effect transistor is then used in its regular circuit operated. The electrode 1 corresponds to the "source", whereas 2 works as a drain electrode.

The situation is reversed if there is a minus voltage at ± U "b. Then the electrode takes over 2 shows the function of a source electrode and electrode 1 that of a drain electrode. As a result of now to the Negative voltage reaching diode D is blocked and the voltage at gate electrode 3 is increased that between it and the electrode 2 there is now that bias voltage that is present when the polarization is reversed the supply voltage between electrodes 3 and 1 was present.

However, nothing changes at the output of the preamplifier, since the arrangement there is essentially symmetrical is what is achieved by equating the resistances R ,, Rc.

The one in Pig. The circuit arrangement shown in FIG. 2 differs from that of FIG. 1 in that a fixed voltage source is switched on instead of the diode D in the auxiliary circuit. This, _{denoted by Uj 1} , can be taken from a battery, for example. The auxiliary circuit is therefore composed of the series connection of the voltage source Uj ₁ with the ohmic resistance R «.

Resistor R, is a common part of the auxiliary circuit and the voltage divider circuit. The bias voltage for the gate electrode 3 of the field effect transistor drops across it. Depending on how the polarity of the supply voltage UJ ₃ turns out, the current flow through the resistor R 1 will increase or decrease, so that here too the gate electrode 3 definitely receives the correct bias voltage. This is shown in Fig. 2 with U «. / _

net and lets out

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R ₃ (U _n R ₁ ± u _b R ₂ )

^u G1 / 2 ~ R ₅ R ₁ + R ₂ R ^ R ₁ R ₂

calculate which formula by applying the voltage divider equation can easily be derived from the circuit of FIG.

The circuit arrangement shown in FIG. 2 for a bipolar preamplifier for sound receivers, in particular capacitive converter, therefore has the same properties as the circuit arrangement shown in FIG. i.e., the supply voltage can be used regardless of the polar

"Did be connected. Am a preamplifier according to the invention can therefore its supply voltage from one equipped with ρήρ and one with npn transistors Power amplifier or the like. it does not require any special attention when connecting and it is sufficient one type for all cases. Without the measures according to the invention the working point setting would be so unfavorable when changing the polarity that e.g. when using of junction field effect transistors in one case the gate diode path becomes conductive, or when using isolated field effect transistors to leave the operating point outside of the usable range of the characteristic curve field. . gen come.

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Claims (3)

Patent claims: Vorrerstärker for sound receivers, in particular for capacitive transducers, which is equipped with at least one field effect transistor and is arranged in the immediate vicinity of the transducer, characterized in that the _{load resistances R 1} , R _{2 of} the stage in the drain and source circuit are the same or approximately the same, the Output voltage is taken between drain and source and the operating point is set such that the potentials of source and gate are the same or approximately the same for both positive and negative supply voltage, whereby the transmission properties remain the same regardless of the process. 2. Preamplifier according to claim 1, characterized in that that the operating point setting by a polarity-dependent Voltage divider, which preferably consists of three resistors and a diode, with polarity dependent two differently divided DC voltages arise. 3. Preamplifier according to claim 1, characterized in that the operating point setting is superimposed by two DC voltages, one of which consists of an auxiliary voltage independent of the polarity of the supply, e.g. battery voltage, and the other is derived from the supply voltage. 809812/1556 Blank page