JPH0284803A - Broad-band amplifier circuit - Google Patents

Abstract

PURPOSE:To relax dispersion in a feedback capacitor due to the dispersion in a current on individual IC substrate without attenuating a high-pass signal by using a semiconductor coupling capacitor, as the feedback capacitor of a broad-band amplifier circuit, and setting input capacitance by appearance to the amplifier circuit so as to be decreased by an out-fitting resistor. CONSTITUTION:A transistor TRQ1 at an initial stage to which a base current is supplied from bias resistors r1 and r2 is provided at the IC substrate, and a current source S1 is cascade-connected to the emitter of the TRQ1 and a base-grounded TRQ2 to the col-lector side. Also, a TRQ3 that is the output of an emitter follower is connected to the load resistor RL with a current source S2. Also, a TRQ4 for voltage-current conversion which applies capacitive positive feedback is provided, and a collector resistor R1 and an emitter resistor RE are connected to the TRQ4, and the coupling capacitor Cj of a diode on which collector output is reverse biased is positive-fed back to the TRQ1 as the feedback capacitor, and the input capacitance by appearance to the amplifier circuit can be decreased by adjusting the out-fitting resistor R2.

Description

[Detailed Description of the Invention] [Industrial Application Field] The present invention relates to a wideband amplification circuit useful in amplifying a wideband signal component with a high S/N [Summary of the Invention] A wideband amplification circuit of the present invention By positively feeding back the output signal of the amplifier to the input side using a capacitor, the input capacitance added to the input terminal of the amplifier circuit is canceled, and the input capacitance added to the amplifier circuit is apparently reduced, and the impedance element of the amplifier circuit is The characteristics of the amplifier circuit, which become unstable due to variations in the current, can be easily stabilized by adjusting the external resistor.

[Conventional technology]

For example, a first-stage amplifier circuit that reproduces a video signal is required to have high S/N and wideband gain characteristics in order to amplify a highband video signal.

Figure 2 shows an example of the circuit of the first stage amplifier of a magnetic head that reproduces a video signal. The transistor Q1 is inputted to the base electrode of the transistor Q1.

The emitter of the transistor Ql in the first stage is connected to the current source S1 and is AC grounded by the capacitor C2, and the collector output is connected to the cascade-connected transistors Q2. and is amplified by the emitter follower transistor Q3 and output with low impedance.

Such an amplifier circuit can create wideband amplification characteristics by taking advantage of the advantages of a common emitter amplifier using the transistor Q+ and the characteristics of a common base amplifier using the transistor Q2. If a large transistor is used as the first stage transistor Q+, the input capacitance IJCo on the base side increases.

Therefore, the resonance frequency with the magnetic head MH decreases, and the level of the high frequency reproduction signal decreases.

Also, due to variations in the inductance of the magnetic head, the resonance characteristics on the input side vary and the DG and Dr characteristics deteriorate.
There was a problem that mass production was affected.

[Problem that the invention seeks to solve]

Therefore, in order to reduce the input capacitance added to the first stage transistor (particularly the base-emitter distance 1c8E, the capacitance of the lead wire, etc.), it has been considered to apply capacitive positive feedback.

Figure 3 (5L) shows the input capacitance H1 of the positive phase amplifier G with a gain of A.
This figure shows a diagram of the principle behind applying capacitive positive feedback to reduce tCo.

In this diagram, input content! If the current flowing through Go is 11 and the current flowing through feedback capacitance C is 12, then the angular velocity is ω
For the input voltage e, 11=jωCO・el 12=j ωCj(e+ −eo) ”°”
(1) holds true. If the gain of amplifier G is A, then C6/
Since el = A, the current i flowing from the input side is i = il + iz = jω (Co + C; (1-A
))e+・・・(2) In the above equation (2), Go+Cj(1-A) ” Ci ・=・・
'(3), then i=JωC1el, so the third
The circuit shown in FIG. 3(b) is apparently equivalent to an amplifier circuit in which the capacitance C1 above is added to the input side, as shown in FIG. 3(b).

In this circuit, if the gain A of amplifier G is A>1, Co =
If the feedback capacitance cj and gain A are selected so that ACj is satisfied, ideally C5=O, and the initially added input capacitance Co can be canceled.

However, in such a circuit, when the apparent input capacitance Ct becomes a negative value, it immediately enters an oscillation state, so
In particular, when it is constructed using an IC circuit, the amount of feedback varies due to variations in the current value of the IC circuit, variations in impedance elements, etc., resulting in an extremely unstable circuit, making it difficult to put it to practical use.

[Means for solving problems]

The present invention has been made in view of these problems.
When configuring a wideband amplifier circuit using an IC circuit, a feedback capacitor for canceling the input capacitance is added using a junction capacitor made of a diode, etc., and at the same time an external resistor is provided to set the amount of feedback, and this resistance value is At the same time, the variation in the current of the transistor is also reduced.

[Effect]

One external resistor is installed on the IC board that constitutes the wideband amplifier circuit, and this external resistor sets the amount of positive feedback depending on the resistance value, and at the same time reduces the variation in the input capacitance that has been added from the beginning. Therefore, the apparent input capacitance can be stably reduced, and the mass production efficiency of IC-based wideband amplifier circuits can be increased.

〔Example〕

FIG. 1 shows a block diagram on an IC board of a wideband amplifier according to an embodiment of the present invention, in which a magnetic head MH, a matching transformer T, and a coupling capacitor C1 are connected to an input signal terminal Tl.

Bias resistor rllr;! on the IC board. The first stage common emitter transistor Q! is supplied with base current by Q! A current source St is provided at the emitter of this transistor Ql. Further, a capacitor C2 for grounding the AC signal is externally connected to the emitter, and a common base type transistor Q2 is connected in cascade to the collector side.

An emitter follower output transistor Q3 is connected to the load resistor RL of the transistor Q2 together with a current source S2, so that an amplified signal is outputted at a low impedance.

Transistor Q4 represents a current-voltage conversion transistor for adding capacitive positive feedback, and emitter resistor RE
and collector resistor R1 are connected. The collector output is positively fed back to the input terminal TI of the first transistor Q1 using the reverse biased junction capacitance c3 of the diode D as a feedback capacitance.

Note that R2 is an external resistor used to set the amount of feedback to a predetermined value and appropriately adjust capacitance changes due to variations in current value, as will be described later.

The input capacitance Co of the first stage transistor Q1 of this broadband amplifier circuit is mainly the base-emitter junction capacitance CBE.
and the distribution capacity of the lead wire connected to the outside −Qk
It is the sum of CR and collector-base capacitance cec.

However, the collector-base capacitance cec has a gain of approximately 1 of the transistor Ql@body, and is hardly affected by the Miller effect.

By the way, the junction capacitance CFB of transistor Q+.

CBC varies from IC substrate to IC substrate, and particularly changes depending on the emitter current Iε (collector current) of the transistor Q+.

Therefore, the overall human power capacity of transistor Q+ is C
If an, then C1n=CG +Q6ΔI t −・・−・
-(4) However, ΔIE=change in emitter current α=proportionality constant.

In the circuit shown in Fig. 1, let A be the gain from the input terminal T1 to the collector output point ■ of the transistor Q4, and let CJ be the junction capacitance due to the diode D. The appearance is the input board above,
iIC+ is C+ = Co+aΔI t◆(1-A) Cj・-”(
5) A) Since it can be set as l, C1=C6+aΔIE −ACJ ・・・・”(
θ) Here, the gain A is the transistor Q1. In Q2,
If the emitter resistance is re, then Rt/re. (Transistor Q3 appears to have a gain of 1.) Also, the emitter resistance re is VH/IE (VT = thermal voltage)
, and since VT may be set to 26 mV at room temperature, r
It appears that e = 26/Ir (mA) Ω.

therefore, ・・・・・・(7) becomes.

By the way, since there is little change in the relative value of resistance on an IC board, it is easy to set RE = RL, and the above (7)
) can be written as (8).

Also, within the IC board, the current and resistance variations are inversely proportional to each other, so the emitter current I of transistor Q1
The change in E and the change in resistance R1 cancel each other out, and the term (1) of the above equation (8) can be set to a constant value regardless of IE.

However, around (2) in equation (8), since the resistor R2 is externally attached, the change in IE is Δ1. This Δ
Considering the changes due to IE, the above equation (8) becomes... (8) Therefore, this equation (9), the above equation (6), and the above equation (7)
) formula, the apparent upper input capacitance C1 is...
10) Therefore, the condition for setting C1=0 is as follows.

Actually, since there is a risk of oscillation in the case of the above equation (11), it is preferable to set the value of the external resistor R2 so as to avoid the possibility of oscillation.

In this equation (12), the diode junction capacitance fl CJ
Since the variations in the input capacitance COε*C8G of the transistor Ql and the input capacitance COε*C8G of the transistor Ql are the same, the above equation (12-1) is stable even against temperature fluctuations, and the apparent input capacitance Ci is set to an extremely small value. can do.

Also, fluctuations due to variations in the current of the IC board are explained in (1) above.
This can be solved by establishing equation 2-2).

〔Effect of the invention〕

As described above, the present invention uses semiconductor junction capacitance as the feedback capacitance, and has a configuration in which the apparent input capacitance of the amplifier circuit can be set to be small using one external resistor. High-frequency signals can be amplified without attenuation, and since the feedback capacitance alleviates variations in current generated in individual IC boards, the operation of the amplifier circuit can be stabilized.

[Brief explanation of drawings]

Fig. 1 is a circuit diagram showing a conventional example of the first stage amplifier of a magnetic head, Figs. 2(a) and (b) are explanatory diagrams for reducing the apparent input capacitance, and Fig. 3 is a circuit diagram of a conventional example of the first stage amplifier of a magnetic head. FIG. 2 is a circuit diagram showing an example. In the figure, Ql is a first-stage transistor, D is a junction capacitance, R2 is an external resistor, St, and 37 are a current source. Figure 2 Figure N3 Teitouri Neho official text (method) % formula % Display of case Patent application No. 62-315086 2゜Name of invention Broadband amplifier circuit 3゜Relationship with the case

Claims (1)

[Claims] A first-stage transistor whose emitter has low impedance and is AC grounded, an amplifier cascade-connected to the output side of the transistor, and an output signal of this amplifier that is connected to an external resistor consisting of a first resistor and a second resistor. partial pressure,
A junction capacitor that provides positive feedback to the input of the first-stage transistor is provided on the IC substrate, and the apparent input capacitance of the first-stage transistor is reduced by adjusting the second external resistor. Wideband amplifier circuit.