DE1117663B - Transistor AC amplifier - Google Patents

Description

GERMAN

PATENfMfT

N16352 IXd / 21a ⁴

REGISTRATION DATE: MARCH 5, 1959

NOTIFICATION OF REGISTRATION AND ISSUE OF
EDITORIAL: NOVEMBER 23, 1961

The invention relates to a transistor AC amplifier in which between the input terminals a certain impedance is effective.

In tube amplifiers, the cross modulation is known to be caused by the anode current the tube is not linear with the grid voltage. The cross-modulation products consequently generated in the anode current one has tried to eliminate, among other things, that in the cathode circuit of the tube a Low frequency filter was added. The result of the curvature of the characteristic curve of the tube above it Filter generated low frequency voltage was used for counter modulation to avoid the undesired Reduce cross modulation.

According to investigations on which the invention is based, it turns out that the in a transistor AC amplifier Cross modulation generated mainly by the internal resistance of the AC voltage source - which plays no role in tube amplifiers in this regard - is influenced.

In the case of a transistor AC amplifier of the type mentioned at the outset, a substantial improvement can thus be achieved can be achieved if, according to the invention, the impedance to reduce the cross modulation approximately fulfills the condition:

- = 3/2

Z ₀ +

Zq - \ -

Z ₀ -f-

al

preferably approximately equal to 1, where Z _{0 is} the internal transistor impedance measured between these input terminals, Zp is the impedance value of the external impedance connected between the input terminals measured for low frequency, Z _{0 is} this value measured at a frequency equal to the difference between the frequency of the useful oscillations and that of an interfering oscillation and Z _{8 represents} this value measured at the sum of the latter frequencies and a _t , a ₂ and a _{3 are} the coefficients of the series

i =

v ² + a ₃ v ³

the transistor input characteristic (transistor input current i as a function of the voltage v applied between its input electrodes).

It is known per se to arrange an additional resistor in the transistor input or the To carry out control of a voltage source exhibiting a considerable internal resistance. Due to the special dimensioning of the resistors in the input circuit, however, according to the invention, a Transistor AC amplifier

Applicant:

N. V. Philips' Gloeilampenfabrieken, Eindhoven (Netherlands)

Representative: Dipl.-Ing. E. E. Walther, patent attorney, Hamburg 1, Mönckebergstr. 7th

Raphael Isidoor Gabriel Bosselaers

and Gerardus Rosier, Hilversum (Netherlands), have been named as inventors

substantial improvement in terms of the occurrence of cross-modulation can be achieved.

The invention is explained in more detail with reference to the drawing.

Fig. 1 illustrates an amplifier according to the invention, e.g. B. an RF or IF amplifier in a receiver for amplitude-modulated oscillations;

Fig. 2 illustrates the cross modulation factor obtained with this amplifier .Oür different Values of the impedances connected between the input terminals of the transistor.

In the amplifier circuit according to Fig. 1, amplitude-modulated useful oscillations of a source ß! with internal resistance 1 to a resonant circuit 3 matched to these oscillations and subsequently, optionally after stepping down, fed to the input terminals of a transistor 4 and amplified. The amplified vibrations are taken from the output circuit 10, which is matched to the useful frequency. Unwanted (interfering) vibrations, e.g. B. originate from a neighboring transmitter, also reach the input circuit and are shown in FIG. 1 as a second source e ₂ . It can be seen that the output current supplied by transistor 4 is not only proportional to these two oscillations, but that this current also contains a component which is caused by cross-modulation of these two oscillations and is in the useful frequency band.

If one assumes that the transistor input current increases practically exponentially with the voltage effective at the input terminals, one finds

109 740 / 37D

for the cross modulation factor K an expression that is approximately independent of the amplitude of the useful oscillation, approximately proportional to. Square of the parasitic oscillation amplitude and further proportional to a factor

1 -

■ + ■

Z ₀ -j-

Z ₀ - \ -

(1)

where Z _{0 is} the internal transistor impedance measured between these input terminals, Z _{v is} the impedance value of the external impedance connected between the input terminals, measured for low frequency (the highest modulation frequency of the useful oscillations), Z _v this value measured at a frequency equal to the difference between the frequency the useful and interfering vibrations and Z _{8 represents} this value measured at the sum of the latter frequencies.

The cross modulation factor K can thus be made practically zero by suitable dimensioning of the impedance connected between the input terminals of the transistor. A simple method consists in that the resonance circuit 3 is connected to the input terminals of the transistor 4 via a resistor 5. The switching elements 7, 8 and 9 used to generate the bias voltage are dimensioned so that they can be disregarded. The impedance of circuit 3 is almost negligible for the three frequencies in question. Even if the isolating capacitor 6 has a negligible impedance for these frequencies, the impedances Zp, Z ₈ , Z _{8 are} each given by the value of the resistor 5. The desired condition is thus fulfilled if the resistor 5 is half of the input resistance R _{0 of} the transistor 4.

In Fig. 2, the curve α shows the cross modulation factor K as a function of the value of the resistor 5 when a capacitor 6 of 250 ^ F is used. With a capacitor 6 of 25 μΤ? the curve b is found with a much less pronounced minimum. With a capacitor 6 of 0.5 \ xB , a minimum is hardly perceptible in the K-Kxave. By connecting a self-induction in series with the capacitor 6, the curve c is obtained. Although the switching elements 6 and 7 could be omitted, but because the setting to the minimum value of the factor K is quite critical, a sufficient operating point stabilization z. B. by a sufficiently large value of the resistor 7 and / or by suitable temperature dependence of the resistor 8 is recommended. The resistor 5 itself can also have a temperature dependency corresponding to the temperature-dependent input resistance of the transistor.

Of course, similar effects can be achieved by differently dimensioning and designing the network connected between the input terminals of the transistor 4. So z. For example, the resistor 5 can be bridged by a small capacitor which is only permeable to the sum frequency of the useful and interfering oscillations (the difference between the useful and interfering frequencies is generally relatively small). In this case, Z _s = 0 and the value of the resistor 5 should be exactly the same as the input resistance R _{0 of} the transistor 4. In the first case (without the bypass capacitor) the internal resistance 1 of the source 1 has to be adjusted to 3/2 R ₀ and a gain loss of about 3.5 db is caused as a result of the resistance 5, in the second case the resistance 5 is set to R ₀ adjust and the gain loss is about 6 db.

The transistor can optionally also be emitter operate. In general, the input current 1 of the transistor as a function of the between voltage applied to its input electrodes through the series

i = a _x ν + a ₂ v ² + a _s v ³ ...

being represented.
If the exponential ratio

holds (where J ₀ , q, Je and T represent constants), one finds for

- Ai - _L and for ^aiUs - 2/3

«1 =

kT

In general, the number 1 in formula 1 must be replaced by 3/2 - ^^ -.

Claims (4)

Patent claims: 1. Transistor AC amplifier, in which between the input terminals a certain Impedance is effective, characterized in that the impedance to reduce the cross modulation approximately fulfills the condition: Z ₀ + Zp + ■ Z ₀ + Z ₈ = 3/2 preferably approximately equal to 1, where Z _{0 is} the internal transistor impedance measured between these input terminals, Zp the impedance value of the external impedance connected between the input terminals measured for low frequency, Z "this value measured at a frequency equal to the difference between the frequency of the useful oscillations and that of an interfering oscillation and Z _{8 represents} this value measured at the sum of the latter frequencies and O ₁ , a _% and a _{3 are} the coefficients of the series i = O ₁ ν + Ct ₂ τ ² + «3 v ³ ... the transistor input characteristic (transistor input current i as a function of the voltage v applied between its input electrodes). 2. Amplifier according to claim 1, in which the useful oscillations via a resonance circuit to the transistor are supplied, characterized in that in series between the resonance circuit and the Transistor input terminals a non-decoupled resistor is switched on, the value of which is approximately half of the transistor input internal resistance is set. 3. Amplifier according to claim 1, in which the useful vibrations are fed to the transistor via a resonance circuit, characterized in that that in series between the resonance circuit and the Transistor input terminals a resistor that is decoupled for the sum of the useful frequency and the interference frequency is switched on, the value of which is approximately the same as the internal resistance of the transistor input. 4. Amplifier according to claim 2 or 3, characterized in that the resistance is accordingly the internal resistance of the transistor input fluctuates with temperature. Documents considered: German Patent No. 826148; “Electronics, Eng. Ed. ”, March 1958, pp. 176-190. 1 sheet of drawings