JPS622721B2 - - Google Patents

Description

[Detailed description of the invention] The present invention relates to a preamplifier.

For example, in magnetic recording and reproducing apparatuses, preamplifiers as shown in FIGS. 1 and 2 are conventionally used for amplifying reproduced signals. Figures 1 and 2
In the figure, 11 and 12 are the main bodies of the preamplifier, which are formed into integrated circuits (ICs), for example. ，，
, are external terminals of the IC. T ₁₁ is a transformer,
_C11 is the resonant capacitor, _R11 is the damping resistor,
C ₁₂ is a coupling capacitor, Q ₁₁ , Q ₁₂ , Q ₁₃ , Q ₁₄ ,
Q ₁₅ is a transistor, E ₁₁ , R ₁₂ is a transistor Q ₁₁
The voltage source, resistor, and
_E12 is the base bias voltage source of transistor _Q12 ,
E ₁₃ is the base bias voltage source of transistors Q ₁₁ and Q ₁₄ , R ₁₃ is the load resistance, R ₁₄ is the emitter resistance of transistor Q ₁₃ , R ₁₅ , R ₁₆ , and D ₁₁ are the transistor Q ₁₅
The resistor, diode, R ₁₇ that forms the base bias circuit of transistor Q ₁₅ is the emitter resistance of V ₁₁
is the power supply.

First, to explain what is shown in FIG. 1, the reproduced signal V reproduced by a magnetic head (not shown)
_i is supplied from the secondary side of transformer T ₁₁ to transistor Q ₁₁ via coupling capacitor C ₁₂ . It is then amplified by transistors _Q11 and _Q12 and output from the emitter of transistor _Q13 . V ₀ is an output signal and is supplied to a subsequent stage signal processing circuit (not shown).

By the way, in the above configuration, the grounding terminal, the coupling capacitor C ₁₁ , the grounding side of the damping resistor R ₁₁ and the grounding side of the secondary winding of the transformer T ₁₁ are grounded at one point to avoid induction. Furthermore, the circuit pattern of the input current loop for the reproduced signal V _i around the IC is designed so that the area of this input current loop is as small as possible.

However, such a configuration has the following drawbacks. That is, the preamplifier main body 11 is
When integrated into an IC, the signal processing circuit connected to the output side of the main body 11 is often also integrated into an IC, and in this case, the ground terminal is commonly used for both circuits. Therefore, the operating current of the signal processing circuit as described above also flows through the ground terminal, and the line from the emitter of the transistor Q ₁₃ to the ground terminal is connected to the operating current of the preamplifier body 11 and the operating current of the signal processing circuit. It becomes a common impedance. For this reason, single point grounding configuration as mentioned above,
If a configuration is adopted in which the area of the input current loop is made as small as possible, there is a risk that the frequency characteristics of the preamplifier may deteriorate or the amplifier may oscillate.

The circuit shown in Figure 2 solves the above-mentioned drawbacks of the circuit shown in Figure 1 by configuring the input section of the signal from the transformer T ₁₁ as a differential input type using transistors Q ₁₁ and Q ₁₄ . It is something.

However, in such a configuration, the spread base resistance of the transistor _Q14 enters the input current loop of the reproduced signal V _i in series, which increases the noise signal in the circuit and deteriorates the NF (noise figure). .

This invention was made in order to deal with the above-mentioned circumstances, and it is possible to reduce the deterioration of the frequency characteristics of the preamplifier by integrating the ground line of the preamplifier and the ground line of the signal processing circuit provided after the amplifier. The purpose of the present invention is to provide a preamplifier that can prevent the risk of oscillation and deterioration of the NF of the preamplifier due to the base spread resistance of the signal input section, and is suitable for IC implementation.

An embodiment of the present invention will be described in detail below with reference to the drawings. In Figure 3, T ₂₁ is 1 in the transformer.
A reproduction signal of voltage V _i is applied to the next winding L ₁ from a magnetic head (not shown). Secondary winding of this transformer T ₂₁
One end of L ₂ is connected to a terminal of the preamplifier main body 21 formed into an IC, and the other end is connected to the terminal via a coupling capacitor C ₂₁ . C ₂₂ and R ₂₁ are a resonant capacitor and a damping resistor, respectively. The terminal is connected to the base of a transistor Q ₂₁ whose emitter is connected to ground, and the emitter of this transistor Q ₂₁ is connected to the terminal and also connected to the emitter resistor.
Connected to the ground terminal via R ₂₂ . E ₂₁ is the base bias voltage source of the transistor Q ₂₁ , which is inserted between the ground terminal and the terminal, and this terminal is connected to the other end of the secondary winding L ₂ , for example, via the base bias resistor R ₂₃ . . Q ₂₂ is a transistor with a common base connection, and the transistor Q ₂₁
Together, they form a cascode amplifier. In other words, the emitter of transistor Q ₂₂ is
Connected to the collector of Q ₂₁ , the collector is a load resistor
Connected to the power terminal via _R24 . E ₂₂ is the base bias power supply for transistor Q ₂₂ .
The collector of transistor Q ₂₂ is also connected to the base of transistor Q ₃₃ with an emitter follower connection. The amplified output V ₀ of the preamplifier is obtained from the emitter of this transistor Q ₂₃ , and its emitter resistor R ₂₅ is connected to the ground terminal. V ₂₁ is the power supply.

Note that the voltage source E ₂₁ may be, for example, a constant voltage source equal to the DC component of the output signal V ₀ or may be controlled to follow fluctuations in the DC component of the output signal V ₀ in order to stabilize the voltage gain. It may also be a voltage controlled voltage source. Further, the voltage source E ₂₂ may be a constant voltage source equal to the voltage of the power supply Vc.c., or it may be controlled to follow fluctuations in the voltage of this power supply Vc.c. in order to stabilize the voltage gain. It may also be a voltage controlled voltage source.

The operation in the above configuration will be explained. Transformer T ₂₁
The reproduced signal V _i induced in the secondary winding L ₂ appears between the base and emitter of the transistor Q ₂₁ . This causes the base current of transistor Q ₂₁ to flow,
This base current is amplified by this transistor Q ₂₁ and becomes a collector current, which is then passed through the transistor Q ₂₂
The signal flows through the load resistor _R24 , and is derived from the emitter of the transistor _Q23 as an output signal _V0 .

By the way, by forming a cascode amplifier with transistors Q ₂₁ and Q ₂₂ as described above, the voltage gain from the base to the collector of the transistor decreases, and the voltage gain from the collector to the collector of transistor Q ₂₁ due to the capacitance between the collector and base of transistor Q ₂₁ decreases. The amount returned to the base is reduced. On the other hand, if there is no transistor Q _{23 ,} the gain in the high frequency range of the preamplifier will drop due to the Miller integration effect, and there is a risk of oscillation. This increases the input capacity.

Here, the effects of this embodiment, which will be described in detail below, will be explained. As is clear from the previous explanation, the secondary winding
Since the reproduced signal V _i induced in L ₂ does not flow to the ground terminal, even if the operating current of the signal processing circuit connected after the preamplifier flows to the terminal, it will not affect the reproduced signal V _i Therefore, problems such as deterioration of the frequency characteristics of the preamplifier and oscillation do not occur.

Furthermore, since the resistor R ₂₂ is inserted between the emitter of the transistor Q ₂₁ and the ground terminal, that is, the reference potential terminal, the voltage gain G ₁ for the terminal or the input signal from the terminal is approximately expressed by the following equation (1).

G ₁ = αR _b /r _e1 +R _a ...(1) However, α: Current increase rate when the base of the transistor is grounded, α = h _fe /1 + h _fe , I _E = αI _C , I _E = h _fe I _B (I _B , I _C , I _E are transistor base, collector, and emitter currents R _a , R _b : Resistance values of resistors R ₂₂ and R ₂₄ r _e1 : kT/q | I _E1 | k...Boltzmann constant T... Absolute temperature q...Electronic charge I _E1 ...Emitter current of transistor Q ₂₁ Also, voltage gain G ₂ as seen from both ends with the terminal
is approximately expressed by the following equation (2).

G ₂ = αR _b / r _e1 = [G ₁ ] R _a = 0 ... (2) Therefore, from equations (1) and (2), if R _a ≫ r _e1 , then G ₁
≪It can be G ₂ . In other words, if R _a ≫ r _e1 in the circuit configuration shown in Fig. 3, this circuit configuration is more resistant to common mode noise signals than the first circuit configuration, and is susceptible to deterioration of the frequency characteristics of the circuit and oscillation. You can avoid dangerous objects that may cause injury.

Further, in the circuit modification shown in FIG. 1, if an overvoltage is applied to the terminal, an overcurrent will flow to the base of the transistor _Q21 , and the base-emitter junction will be destroyed. On the other hand, the third
In the circuit configuration shown, resistor R ₂₂ limits the current flowing into transistor Q ₂₁ , so this transistor
There is no possibility that the base-emitter junction of Q ₂₁ will be destroyed. Also, resistor _R22 forms a current feedback bias circuit, so the transistor
It has the advantage that the bias of Q ₂₁ is stabilized.

In addition, in the circuit configuration shown in FIG. 2, as mentioned above, the base spread resistance of the transistor _Q14 enters in series with the input current loop of the reproduced signal _Vi , increasing the noise signal in the circuit and increasing the NF. However, in the circuit configuration shown in Fig. 3, if the value R _a of the resistor R ₂₂ and the value R _b of the resistor R ₂₄ are set to the same level, the level of the noise signal becomes a problem. do not have. That is, in Fig. 2, the output noise signal level e _rb ² due to thermal noise generated by the base spread resistance r _b of the transistor Q ₁₄ and the resistance R ₁₃ (the resistance value of this resistance R ₁₃ is equal to that of the resistance R ₂₄ ) ) to the output noise signal level e _Rb ² (e _rb ² /e
_Rb ² ) is abbreviated e _rb ² /e _Rb ² = α ² _rbRb / r _e2 ² ...(4
) However, r _e2 : kT/q|I _E2 | I _E2 ...emitter current of transistor Q ₁₄ Also, it is expressed as h _fe ≫1, that is, α≒1. On the other hand, the output noise signal level e _Ra ² due to the thermal noise generated by the resistor R ₂₂ in FIG.
and the noise signal level output by the resistor R ₂₄ e _R
a ² /e _Rb ² is abbreviated e _Ra ² /e _Rb ² = α ² _RaRb / ( _re1 + _Ra
) ² ...(5), so when r _e1 = r _e2 = r _e, equation (4) and (5)
The ratio of the formula is e _rb ² /e _Ra ² = r _b /R _a (1+R _a / _re )
² ...(6) It is expressed as. Equation (6) is the minimum value 4 _r when R _a = r _e
b Take /re. Usually, 4 _rb > re, so R _a =
Even if r _e is selected, the noise signal will not increase more than the circuit shown in FIG. However, normally there is no need to set R _a = r _e ; for example, R _a = R _b , R _a ≫
As can be seen from equation (5), if r _e , the noise signal caused by the resistor R ₂₂ can be reduced to a value approximately equal to the noise signal caused by the resistor R ₂₄ .

As described above, according to the present invention, by integrating the ground line of the preamplifier and the ground line of the signal processing circuit provided at the rear stage of this amplifier, there is a risk of deterioration of the frequency characteristics of the preamplifier, oscillation, and the risk of signal It is possible to prevent deterioration of the NF of the preamplifier due to the base spread resistance of the transistor in the input section, and it is possible to provide a preamplifier suitable for IC implementation.

[Brief explanation of the drawing]

FIG. 1 is a circuit diagram showing an example of a conventional preamplifier, FIG. 2 is a circuit diagram showing another example, and FIG. 3 is a circuit diagram showing an embodiment of a preamplifier according to the present invention. . T ₂₁ ...Transformer, C ₂₁ ...Coupling capacitor, Q ₂₁
……transistor, E ₂₁ ……base bias voltage source, R ₂₃ ……base bias resistor.

Claims (1)

[Claims] 1 A transistor whose base is connected to one terminal of an inductive element to which an input signal is induced and whose other terminal is connected to its emitter via a capacitor, and which is inserted between the emitter of this transistor and a reference potential terminal. and a base bias circuit of the transistor inserted between either terminal of the inductive element and a reference potential terminal, and configured to extract an amplified output of the input signal from the collector side of the transistor. A preamplifier characterized by: