JPS6329846B2 - - Google Patents

Description

DETAILED DESCRIPTION OF THE INVENTION The present invention relates to improvements in low input impedance broadband preamplifiers.

When amplifying a signal from a signal source with low output impedance and large capacity over a wide frequency band, a low input impedance wideband preamplifier is generally used. Regarding this amplifier, the inventor previously filed Japanese Patent Application No. 56-73926.
-188117 Publication), by using two pairs of phase-reversing transistors, it is possible to achieve stability against temperature changes, and also to create a wide-band, low-noise preamplifier with low power consumption and extremely low input impedance. Disclosed.

However, this preamplifier is equipped with an emitter follower at the final stage to keep its output impedance low, and when applied to something that does not require an output signal with such a low output impedance, , the emitsuta holowa has no choice but to become unnecessary.

The present invention is a low input impedance type wideband preamplifier that can stabilize the entire amplifier against temperature changes without using such an emitter follower, and whose amplification characteristics are superior to those of the prior application. The purpose is to provide

That is, in this invention, a common-base amplifier is directly coupled to the first stage, a common-emitter amplifier is directly coupled to the second stage and the final stage, and the first stage amplifier is an NPN transistor, and the second stage and final stage amplifiers are directly coupled to each other. of
Each is composed of PNP type transistors, and a temperature compensation element that also serves as a bias supply is incorporated into the emitter circuit of the second stage amplifier, and the emitter of the final stage amplifier is directly connected to the emitter of the first stage amplifier, and the output is output from the collector of the final stage amplifier. The present invention relates to a low input impedance type wideband preamplifier for obtaining a signal.

This invention eliminates the emitter follower at the final stage in the prior application and obtains an output signal directly from the third stage amplifier immediately before it, and by eliminating the emitter follower at the final stage, the thermal effect of the entire circuit is reduced. The decrease in stability is compensated for by incorporating a temperature compensation element that also serves as a bias supply into the second stage amplifier.

Hereinafter, a specific configuration of the present invention will be explained in detail based on the drawings.

The figure shows the circuit configuration of an embodiment of the amplifier according to the present invention. This amplifier is configured as a direct-coupled three-stage amplifier, with a common base amplifier 10 in the first stage and a common base amplifier 10 in the second and final stages. Grounded emitter amplifiers 20 and 30 are used. The transistors Q ₁ to _{Q 3} constituting each amplifier 10 to 30 are
The first stage is of the NPN type, and the second and final stages are of the PNP type, and the emitter of the transistor _Q2 is connected to a temperature compensating element _D1 consisting of a Zener diode or the like which also serves as a bias supply. By using so-called phase-reversible transistors of NPN type and PNP type, fluctuations in the collector current of these pair of transistors due to temperature changes are canceled out, and for the remaining PNP type transistor, The temperature compensating element _D1 also cancels out fluctuations in its collector current, so that the entire circuit stabilizes the DC output voltage against temperature changes. In this case, the reason why the NPN type is used in the first stage and the PNP type is used in the second and final stages is due to the limitations imposed by the operating conditions of the entire circuit due to the formation of the current feedback circuit F, which will be described later. , the details of which were explained in detail in the earlier application.

This current feedback circuit F is the first stage transistor.
It is formed by directly connecting the emitter of _Q1 and the emitter of the final stage transistor _Q3 . This is to operate so that all the input current to the first stage amplifier 10 is taken in as the emitter current of the final stage amplifier 30, and thereby, the input terminal 1
The input impedance seen from 1 is further reduced,
Broadband amplification and high signal-to-noise ratio can be achieved.

Each transistor Q ₁ to _{Q 3} is connected to two power supplies V 1 and _{V 2} via a load resistor R ₁ to R ₅ and a Zener diode D ₁ .
It is biased to a predetermined voltage value using a current feedback method.

In other words, the collector of transistor Q ₁ is biased through R ₁ , and the emitter is biased through R ₂ , and the base-emitter voltage at that time is maintained at the transistor-specific base-emitter voltage value because the base is grounded. be done.

On the other hand, the emitter of transistor Q ₂ is biased through the Zener diode D ₁ and the collector is biased through R ₃ , and at this time, there is a voltage drop between the base and emitter of the collector current of Q ₁ due to R ₁ , The difference between the voltage across the terminals of the Zener diode _D1 is supplied.

In addition, the collector of transistor Q ₃ is biased through R ₅ and its emitter is biased through R ₂ in the first stage, and at this time, a voltage drop V due to R ₃ of the collector current of Q ₂ exists between the base and emitter. _C2
is supplied with the difference between the base current of Q ₃ plus the voltage drop across R ₄ and the emitter voltage of Q ₁ .

In this case, since the emitter potential of Q ₁ is fixed as described above, the base potential of Q ₃ needs to be lower than its emitter potential by V _BE3 . Therefore, the resistance values of the load resistors R ₁ , R ₂ , and R ₃ are appropriately selected accordingly.

In this manner, when each of the transistors Q ₁ to _{Q 3} is biased to a predetermined voltage value, the emitter current of Q ₁ is stabilized at a constant value, and the operation is as follows.

In other words, an input current is applied to the input terminal 11 in the direction of the arrow.
When ii is introduced, the voltage drop across R ₂ of ii causes the emitter potential of Q ₁ to rise, thereby increasing Q ₁
The base current of V decreases, which further decreases its collector current and increases the collector potential (see v _c1 ).
This increase causes the base-emitter voltage of Q ₂ to decrease, and its base current to decrease. As a result, a collector current amplified by β (grounded emitter current amplification factor) flows through the collector of _Q2 , causing its collector potential (see v _c2 ) to drop.

On the other hand, since the emitter potential V _E3 of Q ₃ is equal to the emitter potential of Q ₁ , the potential V _E3 increases (see _ve3 ). Therefore, the potential difference V _BE3 between the base and emitter of Q ₃ increases additively, increasing the base current of Q ₃ . Therefore, a collector current multiplied by β flows through the collector of Q ₃ ,
This is output as an output signal from output terminals 13 and 14. In this case, it is clear that its output impedance is higher than that of the prior application.

Further, the emitter current of _Q3 is fed back to the input terminal of the first stage amplifier 10 via the current feedback circuit F, thereby stabilizing the amplification operation, improving frequency characteristics over a wide band, and reducing noise.

Although one embodiment has been described above, it is clear that various modified embodiments can be considered within the scope of the invention. For example, a thermistor or the like can be used as the temperature compensation element, and
Instead of the two power supply system as shown in this embodiment, it is also possible to configure the system with one power supply system.

As explained above, in this invention, an amplifier is constructed using one set of a pair of reciprocal transistors, one set of transistors whose temperature characteristics cancel each other out, and one set of a temperature compensation element. The stability of the DC output voltage against changes can be improved. In addition, since the amplifier according to the present invention is configured as a direct-coupled three-stage amplifier, unlike the prior application which is a direct-coupling four-stage amplifier, the number of circuit elements can be reduced compared to that of the prior-application, and its IC
It can also be done easily and inexpensively. Also,
Since the number of circuit elements is reduced, the power consumption can be reduced compared to the prior application.

[Brief explanation of the drawing]

The figure is a circuit diagram showing one embodiment of a preamplifier according to the present invention. 10...Base grounded amplifier (first stage amplifier), 2
0... Grounded emitter amplifier (second stage amplifier), 3
0... Grounded emitter amplifier (final stage amplifier), D ₁
... Temperature compensation element, Q ₁ to Q ₃ ... Transistor,
13, 14...Output terminal, C...Feedback capacitor, _V1 , _V2 ...Positive and negative power supply voltage.

Claims (1)

[Claims] 1 A common-base amplifier is directly coupled to the first stage, and a common-emitter amplifier is directly coupled to the second and final stages, with the first stage amplifier being an NPN transistor, and the second and final stage amplifiers being PNP transistors. In addition to incorporating a temperature compensation element that also serves as a bias supply into the emitter circuit of the second stage amplifier, the emitter of the final stage amplifier is directly connected to the emitter of the first stage amplifier so that an output signal is obtained from the collector of the final stage amplifier. A low input impedance type wideband preamplifier.