RU2617930C1 - Cascode amplifier of type general drain - general base - Google Patents

Abstract

FIELD: radio engineering, communication.

SUBSTANCE: amplifier contains an n-channel field effect transistor with p-n junction and a p-n-p type bipolar transistor, the emitter of which is short-circuited to the source of the field effect transistor, and the collector through the load is galvanically connected to the supply minus, the drain of the FET is connected to the common wire, wherein the gate of the FET, through the impedance of the input circuit, is galvanically connected to the supply minus, and the base of the bipolar transistor is connected to the bias voltage source.

EFFECT: creating a low-power cascode amplifier with low noise performance, simplifying cascading and gain controlling without the need to select the field effect transistors used.

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Description

The invention relates to electronics and can be used as a basic circuit node in specialized devices for receiving and transmitting radio signals, in circuits of specialized direct current amplifiers (DCTs) and other circuits for converting signals with reduced power supply.

Known circuits for operational amplifiers with field effect transistors paired with bipolar as part of a cascode circuit at the inputs. For example, in the circuit of an operational amplifier (op amp) of Analog Devices type ADA4610 in the first amplification stage as part of a differential amplifier (DC) of a direct current, the circuit of the cascode amplifier (cascode) of the type “Common drain - Common base” (OS-OB), consisting of from an input JFet type field effect transistor with a p-type channel, the source of which is loaded with an emitter junction of an n-p-n-type bipolar transistor. Since both OS-OB cascodes operate in parallel as part of the remote control, their drains are connected together and loaded on an emitter follower. The collectors of npn transistors of both cascodes have a load identical to the branches of the current mirror, which leads to a high gain of the first cascade of op-amps. Such inclusion in the structure of the CTF does not reduce the supply voltage and does not allow to identify the properties of an individual cascade of this type and evaluate its use for the indicated purposes in the field of radio communications.

The known scheme cascode amplifier used in the high frequency amplifier of a direct gain receiver (Journal of Radio, No. 7, 8 for 1981, Y. Stepanyan "Block RF receiver direct amplification"). A cascode amplifier of the OK-OB type consists of two bipolar transistors of different conductivity, the emitters of which are short-circuited, which made it possible to exclude low-impedance resistors from the emitter circuits and gave good sensitivity while reducing power consumption. Since the second transistor of the cascode amplifier has a base ground at a high frequency, the Miller effect is eliminated, the device is stable and not prone to self-excitation. The disadvantages of this circuit can be considered a relatively high lower limit of the supply voltage, the need for additional circuit elements to create the operation modes of both transistors for direct current and communication between stages, the unresolved problem of automatic gain control, as well as the dependence of the input resistance of the cascode on the supply current, and therefore from gain.

A known circuit cascode amplifier type OS-OB (Goroshkov B. I. Radio-electronic devices: a Handbook. - M .: Radio and communications, 1984, S. 95, Fig. 2.43c), consisting of a p-channel field-effect transistor with p-n transition and bipolar transistor n-p-n-type, the base of which is connected to a common wire, and the collector through the load Rn - with a plus power. The source of the field effect transistor is connected to the minus power, and the input voltage is applied to the gate. The Miller effect with this inclusion is insignificant, and the use of a field effect transistor with a pn junction as input allows you to reduce the minimum supply voltage of the circuit. The disadvantage of this scheme is the need for additional elements for creating DC transistors operating modes and dividing DC cascades (these elements are absent in the source, but the circuit is inoperative in their absence; the directory format allows not to show additional elements on the circuits). In addition, the circuit does not provide automatic gain control.

The present invention is directed to the creation of a cascode amplifier with reduced power supply, compatible in supply voltage with modern microprocessors, universal for constructing other special signal conversion circuits without the need to select the used field-effect transistors for cut-off voltage and initial current.

The technical result of the invention is to simplify the cascading and the ability to control the gain using the built-in intelligence in the AGC without complicating the power source.

The indicated technical result is achieved by a cascode amplifier containing an n-channel field effect transistor with a pn junction and a pn-p type bipolar transistor, the emitter of which is short-circuited to the source of the field-effect transistor, and the collector is galvanically connected to the supply minus through the load, and the drain of the zero transistor is connected with a common wire, in which, unlike the prototype, the gate of the field-effect transistor is galvanically connected to the supply minus via the input circuit impedance, and the base of the bipolar transistor is connected to the source of the offset voltage.

The invention is illustrated by drawings, where in FIG. 1 is an electrical diagram of a cascode amplifier type OS-OB; FIG. 2 is an example of cascading the circuit of FIG. one.

The cascode amplifier (Fig. 1) contains an n-channel field effect transistor with a pn junction 1, a bipolar transistor pnp type 2, a load impedance in its collector circuit 3 (resistive, inductive or parallel oscillatory circuit) and an impedance of the input circuit 4.

The amplifier (Fig. 1) works as follows. The gate of the field-effect transistor 1, which serves as the input of the amplifier, is supplied with a source signal with a zero constant voltage component (from the secondary winding of the matching transformer, inductance of the antenna circuit or parallel oscillatory circuit). The load of the cascade with a common drain (source follower) on an n-channel field effect transistor with a pn junction 1 is the emitter-base junction of a bipolar transistor 2 of the pnp type, connected according to the scheme with a common base. Its base is connected to the source of the basic offset Esm. Depending on the voltage of the power source and the choice of the base bias of transistor 2, transistor 1 can operate on any part of its current-voltage characteristic (CVC), up to the beginning of the CVC, where it works as a variable resistance controlled by the gate potential. In any case, the current of transistor 1, and therefore of transistor 2, is determined by the intersection of the load curve (branch of the input characteristic of the stage with a common base) with the corresponding branch of the I – V characteristic of transistor 1. Thus, the gain of the stage is determined by the formula: K = S * Z * α ; where α = 1-1 / β, S is the slope of the I – V characteristic of transistor 1 at the intersection point of the I – V characteristic, Z is the load impedance for the signal frequency, and β is the gain for a small signal over the current of transistor 2. The bias voltage of the base of transistor 2 controls the amplifier mode. An increase in this voltage shifts the operating point of the field-effect transistor 1 to the beginning of the I – V characteristic, decreasing the drain current, gain, and drain voltage. This increases the input and output impedances and decreases the level of intrinsic noise of the field effect transistor. Thus, the proposed cascode amplifier circuit allows by selecting the base bias voltage to reduce the noise characteristics of the circuit.

In FIG. 2 shows an example of a cascade connection of three cascode amplifiers of the OS-OB type. For the first stage, the input circuit impedance is a parallel LC circuit, consisting of capacitance C and inductance L, and load resistance is a parallel connection of the inductance L1 and the input resistance of the next stage. For the second and subsequent stages, the input circuit impedance is a parallel connection of the output circuit of the previous stage and the inductance L2. The load of the last stage is the inductance Li, induced by the load resistance R. As follows from the circuit of FIG. 2, the high input and output impedance of the cascade, as well as the low resistance of the source and emitter junction of the transistors provide the possibility of sectional shielding of cascades with the separation of transistors of one cascade along the low-resistance source-emitter circuit, with the combination of transistors of adjacent cascades in one section of the screen at high-impedance points of output and input . This allows you to easily avoid self-excitation when you need more sensitivity. In the proposed cascading scheme, inductors can be replaced by band-pass LC filters or parallel LC circuits.

The specified technical result is achieved in the invention due to the following.

Since the pn junction of the field-effect transistor is biased by the gate closing potential, the DC voltage component of the collector-emitter of transistor 2 is in the region from its saturation voltage to the cut-off voltage of transistor 1, therefore, with an appropriate choice of transistor 1 type, the OS-OB cascade can work, starting with a supply voltage of 1 V or less in the case of a germanium bipolar transistor 2.

The choice of the operating mode of the circuit using the bias voltage of the base of the transistor 2 allows you to achieve the desired circuit parameters within the type of transistor 1, which can also be simply done by replacing the bias voltage of the base with the voltage of the automatic gain control system. The simplicity of cascading and gain control is demonstrated by the circuit of FIG. 2.

Reducing the noise characteristics of the circuit is achieved by selecting the bias voltage of the base of the bipolar transistor.

The versatility of the circuit node for converting signals is achieved by using the base of transistor 2 as a second AC input. For example, when applying a local oscillator signal to a base with a cascode offset in the nonlinearity region, the circuit operates as an amplitude modulator.

Thus, the proposed cascode amplifier type OS-OB is a universal circuit node for constructing special signal conversion circuits with reduced power supply, with low noise characteristics. EFFECT: simplification of cascading and gain control without the necessity of selecting used field-effect transistors.

Claims (1)

A cascode amplifier containing an n-channel field-effect transistor with a pn junction and a pnp type bipolar transistor, the emitter of which is short-circuited to the source of the field-effect transistor, and the collector is galvanically connected to the supply minus through the load, and the drain of the field-effect transistor is connected to a common wire, characterized in that the gate of the field-effect transistor is galvanically connected to the negative of the power supply through the input circuit impedance, and the base of the bipolar transistor is connected to a bias voltage source.

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