JP5521783B2 - Transimpedance preamplifier - Google Patents

Description

  The present invention relates to a transimpedance preamplifier used in a receiving system using optical communication.

  In a receiving system using optical communication, a preamplifier that converts an optical signal received by a light receiving element such as a photodiode into a voltage signal is required to have a wide dynamic range with low noise. A transimpedance preamplifier having a feedback resistor is generally used in an optical receiver (ROSA: Receiver Optical Sub-Assembly) that satisfies such requirements. A transimpedance type preamplifier is known in which a transimpedance value is changed according to input power in order to secure a dynamic range and a band.

  For example, in Patent Document 1, two parallel-connected field effect transistors (FETs) are used as feedback resistors, a constant voltage is applied to the gate of one FET, and an output is applied to the gate of the other FET. It is disclosed that frequency characteristics and gain characteristics are improved by adjusting a feedback amount by applying a DC voltage corresponding to an amplitude amount. Further, Patent Document 2 includes a decoder that outputs transmission rate information, and controls a feedback resistor and an internal gain resistor (first stage load resistor) by a signal from the decoder, so that an optimum frequency for different transmission rates is obtained. Obtaining properties is disclosed.

JP-A-8-167816 JP 2006-80988 A

  As shown in FIG. 3A, a general ROSA is composed of a light receiving element (photodiode: PD) and a transimpedance preamplifier (TIA). When the frequency band exceeds 10 GHz, The parasitic capacitance Cpd and the like cannot be ignored. In FIG. 3, Q0 is an input stage (first stage) transistor, Q1 is a buffer stage transistor, R0 is a load resistor, R1 is a feedback resistor, and R2 is a resistor. The frequency band of this ROSA is determined by the pole formed by the parasitic capacitance Cpd of the PD and the like and the input impedance Zin of the TIA and the circuit configuration of the TIA itself. Therefore, by changing the input impedance Zin of the TIA, the frequency of the pole formed by the parasitic capacitance Cpd of the PD changes, and the band can be controlled.

It should be noted that the equations showing the characteristics of the TIA circuit shown in FIG. 3A can be expressed as follows.
Zin ≒ R1 / (1 + A0) (1)
Zt = Vout / Iin≈R1 (2)
f ₋₃ dB≈ (√2 · A0) / (2π · R1 · Cpd) (3)
A0 = gm0 · R0 (4)
gm0 = (q · I0) / (k · T) (5)
(A0: TIA open loop gain, f _{−3 dB} : band, gm0: transconductance of transistor Q0, k: Boltzmann constant, q: elementary charge, T: absolute temperature, I0: collector current of transistor Q0)

  From the above formulas (1) and (2), in order to change the input impedance (Zin), it is conceivable to change the transimpedance Zt. However, there is a possibility that an appropriate gain cannot be obtained. In addition to this, adding a circuit for making the transimpedance (Zt≈R1) variable as disclosed in Patent Documents 1 and 2 may increase the parasitic capacitance component, which may be disadvantageous in high-speed operation. is there. Therefore, in order to avoid this, it is desirable to control the band without changing the transimpedance Zt.

From the above equation (3), in order to extend the frequency band (f _{−3 dB} ) while maintaining the transimpedance Zt (ie, without changing the feedback resistor R1), the open loop gain A0 of the TIA is increased. You can see that In order to increase (A0), the mutual conductance gm0 or the load resistance R0 of the transistor Q0 may be increased from the equation (4). In order to increase (gm0), it can be understood from equation (5) that the collector current I0 of the transistor Q0, which is the load current I0 ′, {I0 ′ = I0} in FIG. 3A) should be increased.

  As a method of increasing the collector current I0 of the transistor Q0, as shown in FIG. 3B, a gain boost in which a current source Im is arranged in parallel with the load resistor R0 and current is injected can be considered. Specifically, for example, it is realized using a current mirror circuit as shown in FIG. This current mirror circuit is formed by using, for example, a pair of PMOSFETs (M0, M1), and the current Im from the current source is added to the load current I0 ′ passing through the load resistor R0 to obtain the collector current I0 of the transistor Q0. It becomes a form.

  However, when this current mirror circuit is used, the parasitic capacitance (Cgd) between the gate and the drain of the PMOSFET (M1) and the parasitic capacitance Cdb between the drain and the substrate (silicon substrate forming the TIA) are the band at the X point. Deteriorates the operation of high-speed transmission. Further, when the load resistance R0 is increased in order to increase A0, the parasitic capacitance associated with the resistance is increased, which is disadvantageous for high-speed operation.

  The present invention has been made in view of the above-described circumstances, and an object of the present invention is to provide a transimpedance preamplifier capable of adjusting a frequency band without changing a transimpedance value or a load resistance value.

A transimpedance type preamplifier according to the present invention is a preamplifier that is used in a frequency band exceeding 10 GHz and includes an amplifying unit and a transimpedance unit that feeds back an output signal of the amplifying unit. constant voltage generating means of the power supply voltage supplied to the load resistor (Vcc ') is provided on the same integrated circuit to form a pre-amplifier, said voltage generating means, which is connected to a reference voltage source (Vcc) of It consists of a series circuit of a current source and a resistor provided with a plurality of connection taps, and the power supply voltage of the input unit is adjusted by adjusting the resistance value of the resistor by selecting the connection tap and wire-connecting It is made possible .

  According to the present invention, the frequency band can be adjusted without changing the transimpedance value and load resistance of the TIA and without performing gain boost by current injection. Further, the power supply voltage (Vcc ′) for frequency band adjustment can be generated by voltage generation means provided in the same integrated circuit where the TIA is formed using a common reference voltage source (Vcc). This can be realized without increasing or changing the number of terminals for connection to an external circuit, changing the size of the ROSA package, etc. in the ROSA on which the TIA is mounted.

It is a figure explaining embodiment of this invention. It is a figure which shows an example of the frequency characteristic adjustment result by this invention. It is a figure explaining a prior art. It is a figure explaining the subject of a prior art.

  The outline of the present invention will be described with reference to FIG. 10 is an integrated circuit, 11 is a light receiving element (PD), 12 is a voltage regulator, 13 is a voltage generating means, 14 is a wire, 15a to 15n are connection taps, and 16 is a constant current source. The other reference numerals are the same as those used in the description of FIG.

  The transimpedance preamplifier (TIA) according to the present invention is mainly intended for use in an optical receiver (ROSA) used in a band exceeding 10 GHz. As shown in FIG. 1A, the basic configuration of the TIA itself is the same as that described in FIG. 3, for example, and the current signal Iin photoelectrically converted by the light receiving element (PD) 11 is the first stage of the TIA. Is converted into a voltage signal. Then, this voltage signal passes through the transistor Q1 in the buffer stage, and is output as an output voltage Vout to the next stage amplifier circuit from the connection point between the emitter of the transistor Q1 and the grounded resistor R2. The output is fed back to the input terminal of the transistor Q0 by the feedback resistor R1.

  When the current signal Iin generated by light reception by the PD 11 is input to the base of the transistor Q0 in the input stage, a collector current I0 flows, and the collector current flows through the load resistor R0 as a load current. By the above formulas (3) to (5), the open loop gain A0 is increased by increasing the collector current I0, and the frequency band of the TIA can be expanded. In the present invention, the collector current I0 is adjusted without changing the set values of the load resistor R0 and the feedback resistor R1 by making the power supply voltage (Vcc ′) supplied to the load resistor R0 adjustable. be able to.

The collector current I0 can be roughly expressed by the following equation.
I0≈ {Vreg− (Vbe0 + Vbe1)} / R0 (6)
Here, (Vreg) is a power supply voltage supplied to one terminal of the load resistor R0, (Vbe0) is a voltage between the base and emitter of the transistor Q0, and (Vbe1) is between the base and emitter of the transistor Q1. Is the voltage. Note that the current flowing through the feedback resistor R1 is negligible (for example, the input current is small or the DC input current is neutralized), and the voltage drop at this portion is Suppose not.

  In the above equation (6), assuming that the values of the load resistance R0 and the voltages Vbe0 and Vbe1 are constant, the collector current I0 can be changed by adjusting the power supply voltage Vreg. The power supply voltage Vreg can be adjusted using, for example, the voltage regulator 12 and the voltage generation unit 13. The voltage regulator 12 can be formed by an operational amplifier using a voltage follow circuit, for example. Since this circuit has a high input impedance, it is not necessary to flow a current from the power source side, and since it has a low output impedance, the output current can be taken out. The voltage regulator 12 and the voltage generating means 13 are provided in the same integrated circuit chip that forms the TIA.

As the voltage generating means 13 , a voltage adjusting circuit as shown in FIG. The voltage generating means 13 generates a desired potential in the integrated circuit 10 of the TIA using a reference voltage source (Vcc..., For example, formed by a band gap reference circuit) such as an integrated circuit . consisting series connected plurality of resistors (R3, R4, R5 ··· Rn ) and the constant current source 16.. The plurality of resistors (R3, R4, R5... Rn) may include those connected in parallel in addition to the series connection.

  Then, the voltage Vreg input to the voltage regulator 12 is taken out between the constant current source 16 and the resistor R3. At this time, the output of the voltage regulator 12 becomes Vreg, and Vcc ′ = Vreg. Further, connection taps 15a to 15n are taken out between the resistors, such as a connection tap 15a between the resistors R3 and R4 and a connection tap 15b between the resistors R4 and R5. The series resistor circuit may be configured such that one resistor R is divided at an appropriate interval and a connection tap is drawn from the divided portion.

  Of the connection taps (15a to 15n), a desired connection tap is selected, bonded using a wire 14 such as a gold wire, and dropped to a ground potential such as a ROSA package substrate on which the TIA is mounted. As a result, for example, when the connection tap 15a between the resistors R3 and R4 is selected and grounded by the wire 14, a voltage is generated by the resistor R3 and the current Ireg of the constant current source 16, and this voltage value is the power supply voltage. Vreg. When the connection tap 15b between the resistors R4 and R5 is selected and grounded by the wire 14, a voltage is generated by the resistor (R3 + R4) and the current Ireg of the constant current source 16, which is higher than when the connection tap 15a is selected. A power supply voltage Vreg having a voltage value is obtained. The voltage value of the generated power supply voltage Vreg can also be adjusted by electrically short-circuiting the connection taps (15a to 15n) using a wire or the like.

  FIG. 2 is a diagram showing frequency characteristics of transimpedance Zt (gain) when the power supply voltage Vreg is adjusted using the TIA according to the present invention. The frequency characteristic (a) is, for example, a case where the connection tap 15a in FIG. 1B is grounded (current Ireg flows only through the resistor R3). The frequency characteristic (b) is when the connection tap 15b is grounded (current Ireg flows through the resistors R3 and R4), and the frequency characteristic (c) is when the connection tap 15c is grounded (current Ireg through the resistors R3, R4, and R5). This is the case where

  When the resistance value of the voltage generating means is increased and the power supply voltage Vreg is increased, the load current, that is, the collector current of Q0 is increased, the band is extended by Equation (3), and the frequency characteristic is shifted from (a) to (c). . The frequency band obtained by lowering the transimpedance value on the low frequency side by the specified −3 dB is, for example, 16.5 GHz in the frequency characteristic (a) and 19.5 GHz in the frequency characteristic (c), and the band is adjusted. Is possible.

  The wire 14 can be directly connected to the ground terminal of the mounted ROSA by selecting a desired connection tap (15a to 5n) after forming the TIA on the integrated circuit 10. On the other hand, the use band of TIA may be changed after manufacture, or the characteristics of the TIA may vary slightly during the manufacturing process of TIA. In such a case, after manufacturing the TIA, the collector current I0 can be adjusted by selecting the connection tap of the voltage generating means 13 and adjusting the power supply voltage Vreg, and changing the open loop gain to change the frequency. The characteristics can be adjusted. As a result, it is possible to obtain a uniform quality ROSA without variation or increase the power supply voltage Vreg to extend the high frequency band.

  As shown in FIG. 1A, the voltage regulator 12 and the voltage generating means 13 are not provided in the integrated circuit forming the TIA, but are directly supplied from an external circuit separately from the reference power supply (Vcc). It is also possible to supply as However, in this case, an external connection terminal for the power supply voltage Vreg is required for the ROSA on which the TIA is mounted. For this reason, it is necessary to enlarge the package shape of the ROSA in order to secure a space for increasing the number of terminals, and it is impossible to satisfy the demand for downsizing. Moreover, there is a problem that the increase in the number of terminals does not satisfy the standard specification. Therefore, by providing the voltage regulator 12 and the voltage generation means 13 in the integrated circuit chip forming the TIA, it is possible to prevent the configuration of the ROSA package from being affected.

DESCRIPTION OF SYMBOLS 10 ... Integrated circuit, 11 ... Light receiving element (PD), 12 ... Voltage regulator, 13 ... Voltage generation means, 14 ... Wire, 15a-15n ... Connection tap, 16 ... Constant current source.

Claims (1)

A preamplifier used in a frequency band exceeding 10 GHz, comprising an amplifying unit and a transimpedance unit that feeds back an output signal of the amplifying unit,
A voltage generation means for the power supply voltage supplied to the load resistance of the input section (first stage) of the preamplifier is provided in the same integrated circuit forming the preamplifier, and the voltage generation means is connected to a reference voltage source A power source of the input unit by adjusting a resistance value of the resistor by selecting the connection tap and wire-connecting the selected constant current source and a resistor provided with a plurality of connection taps. A transimpedance preamplifier characterized in that the voltage is adjustable.

Images