DE1275138B - Counter-coupled transistor amplifier - Google Patents

Description

FEDERAL REPUBLIC OF GERMANY

GERMAN

PATENT OFFICE

EDITORIAL

Int. Cl .:

H03f

German KL: 21 a2- 18/05

Number: 1275138

File number: P 12 75 138.1-31 (S 106079)

Filing date: September 23, 1966

Opening day: August 14, 1968

The invention relates to a negative feedback transistor amplifier with an input and output transformer.

It is known that bridge circuits are preferably used when one is dealing with relatively small changes in resistance wants to achieve very large changes in the voltage transfer factor. Instead of one A bridge circuit can also be used with a bridge T circuit in systems that are unbalanced to earth. This has only three connection points and also requires one that is suitable for the transmission area Economy transformer only one series and one transverse resistor. One needs to make the change you want To achieve the transfer factor, only to change one of the resistors. Usually However, the longitudinal and transverse resistance must be changed reciprocally if that Network should have constant characteristic resistances at the input and output. But this is above all then difficult when a network of this type is to be used as an actuator in a control circuit. It is Although possible, single-stage both at the input and at the output for decoupling and adjustment To use amplifiers, however, this effort is often not justified. Also are in German Auslegeschrift 1 188 664 circuits have been proposed in which one of the two resistors is replaced by an active element in such a way that when the other resistance changes the characteristic resistances at the input and output are retained. These circuits behave but otherwise like passive quadrupoles.

The object of the invention is to create an amplifier in which the input and output resistances are constant are adjustable and the gain can easily be adjusted to large attenuations is.

The transistor amplifier is designed according to the invention in such a way that between the input transformer and the input circuit of the amplifier is the bridged series branch of a bridge T-link and that the shunt of this bridge T-member is a variable resistor connected to the output circuit of the amplifier is connected in the feedback sense.

These measures result in an amplifier arrangement in which the specific properties a negative feedback amplifier are advantageously connected to those of a bridge circuit. Furthermore, broadband amplifiers with selective narrow ones can be achieved by the teaching according to the invention Realize damping poles, so that also selective

Counter-coupled transistor amplifier

Applicant:

Siemens Aktiengesellschaft, Berlin and Munich, 8000 Munich 2, Witteisbacherplatz 2

Named as inventor:

Hans Hochrath, 8000 Munich

Amplifiers such as pilot amplifiers, etc. can be constructed in a simple manner. The reinforcement can be influenced by reducing the cross resistance of the bridge circuit to the extent that that with a steep characteristic, large attenuations can also be set. Both Constant characteristic resistances of the desired size can be implemented at the input as well as at the output. Because of the steeper characteristic curves, the amplifier can also be used as a control amplifier. Furthermore, the output power and distortion attenuation correspond to the normal negative feedback, single-stage Amplifier without a bridge T-element with a much higher stability of the circuit.

If one uses any complex two-pole instead of the ohmic resistance, one obtains frequency dependent amplifier. With sufficient negative feedback, broadband input and Output resistances generated in the desired size. By reducing the transverse resistance of the Bridge-T circuit, the gain can be reduced with a large control steepness until finally a damping pole is reached and the output voltage becomes zero. With further downsizing of the cross resistance, the polarity of the output voltage is reversed. The damping now decreases until again Reinforcement is achieved.

Using the exemplary embodiment according to FIG. 1, which, for the sake of simplicity, is a single-stage transistor amplifier shows, and the equivalent circuit diagram according to FIG. 2 the invention is explained in more detail.

The series branch is in a bridge T-circuit with the transformer U ₂ and the resistor R ₁ between the pre-transformer O ₁ and the base of a transistor Tr, while the shunt branch with the variable resistor R, in the collector circuit at an output

handle of the output transformer U _{3 is} connected. One comes to a simple representation of the circuit principle if one first turns to

TO9 590/31 «

the calculation of the gain factor is limited regardless of optimal dimensioning. For this purpose it is assumed that the input voltage U _{e is} induced in the winding W _{2 of} the pre-transformer U ₁ and the transistor is operated as an ideal amplifier in a collector circuit by making the winding W _{6 of} the post-transformer U ₃ and the resistor R ₂ zero. If the bridge circuit with transformer U ₂ is now replaced by an equivalent T circuit, the illustration according to FIG. 2 is obtained.

With the voltages specified there, the circuit principle is easy to overlook. The relationship between input voltage U _e and output voltage U _a can be expressed by the following simple relationship:

Resistors R ₁ and R _{2 influence} the result. However, the formulas for the input resistance and output resistance still have to be taken into account.

U,

2R ₁

U)

This result shows that in the proposed circuit the bridge circuit has lost the character of a passive quadrupole. Because the variable resistor R _r only appears in the numerator, it can be larger than the denominator, which then results in an amplification of the input signal U _e. In addition, this means that when the control resistance is reduced, the transition from the amplification to the attenuation range takes place with a very steep characteristic curve.

To further assess the circuit, it is now necessary to grasp all relationships. In the circuit according to FIG. 1, one has to take into account that the transistor there operates with a combined current-voltage negative feedback and that optimal operating conditions can be set by the transmitter. The expression for the transfer factor A is thus obtained

A =

W ₁ W ₅

(2)

W ₅ + W ₆

-R ₁ -

W ₃ - W ₁ (W ₃ + W ₄ )

2-R ₁

W ₃ +

40

45

As you can see, the characteristics of relationship (1) are retained. But you still have that Possibility, by suitable choice of the transmission ratios of the transformers and dimensioning of the The following applies to the input resistance:

~ Rr.

Wi W ₃ + W ₄ .
The following applies to the output resistance:

M / 2

R). - Ra

W ₅ (W ₅ + W ₆ )

A comparison of these formulas shows that you can get the input resistance, the output resistance as well the reinforcement can largely be dimensioned independently of one another. However, one must not forget that a transistor is used. This also defines limits that are specified in the following rules of thumb are indicated.

(W ₃

R ₁ > rd,

R,

W ₃ +

W ₄ R _r

R ₁ <ß ² approx.

> rd,

Here, the differential emitter diffusion resistance is denoted by rd .

Claims (2)

Patent claims: 1. Counter-coupled transistor amplifier with input and output transformer, characterized in that between the input transformer (U ₁ ) and the input circuit of the amplifier, the bridged series branch of a bridge T-member is and that the cross-branch of this bridge T-member is a variable resistor which is connected to the output circuit of the amplifier in the feedback sense. 2. Counter-coupled transistor amplifier according to claim 1, characterized in that the shunt arm resistor (R _r ) is a frequency-dependent resistor .. Considered publications:
German publication No. 1188 664. 1 sheet of drawings 809 590/318 1.68 O Bundesdruckerei Berlin