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Industry News / Amplifiers / Semiconductors / Integrated Circuits

A Cost-effective 2 W GaAs MMIC Amplifier Solution

November 1, 2002
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GaAs HBT technology has been successfully deployed for low to mid-power MMIC RF amplifiers in recent years. EiC Corp. pioneered the use of an InGaP emitter for RF power amplifiers and first introduced gain block products in 1998. InGaP/GaAs HBT devices have proven to provide better performance and be more reliable than their AlGaAs/GaAs HBT predecessors. Today, other semiconductor manufacturers have followed suit, and InGaP HBT products are superceding AlGaAs.


Recently, InGaP HBT capability has been taken to the next level with the introduction of a series of intermediate power amplifiers. Using the same process applied to already-offered lower power gain blocks, one-half-, one- and two-watt MMIC RF amplifiers are now available. The ECP05x series are 1/2 W, the ECP10x are 1 W and the ECP20x are 2 W amplifiers. Exceeding 1 W capability for a MMIC product is truly a milestone in the industry. Technical data referred to later in this article focuses on the ECP200 2 W MMIC amplifier.

Table 1
ECP-Series Intermediate Power Amplifier Characteristics

Model

P1dB*
(dBm)

Gain*
(dB)

OIP3*
(dBm)

Frequency
(MHz)

ECP050

27

15

45

1800 to 2300

ECP052

27

15

45

800 to 1000

ECP053

28

17

45

2100 to 2700

ECP100

31

12

47

100 to 2300

ECP103

31

12

47

2100 to 2700

ECP200

33

10

49

100 to 2300

ECP203

33

11

49

2100 to 2700

*Typical values as stated on the data sheets

ECP Device Features

The ECP-series amplifiers are cost-effective, and feature a single-stage MMIC design in small outline plastic packaging with built-in ESD protection and the reliability benefits of InGaP HBT technology. The new amplifiers offer significant design flexibility by providing Vde up to 7 V, class A/AB selectivity, temperature compensation bias and power up/down sequencing. In addition, partial input matching circuitry is built on-chip. Table 1 lists the operating characteristics of the intermediate ECP-series.

Temperature Compensated Current Mirror Bias Circuit

Figure 1 shows a block diagram of the amplifier bias circuit. The amplifier is a single-stage design. The output matching circuit is provided off-chip and the collector current is provided through an RF choke of the output matching circuit.

The bias circuit is based on the current mirror principle and provides compensation over temperature for the bias current. Vbias requires a DC bias voltage, usually connected to Vcc, to provide the bias to the current mirror circuitry.

Figure 2 shows measured results for the class A bias condition for the 2 W, ECP200 amplifier over temperature. The RFout and Vbias are at 5 V, and the Icc is measured against Vref as Vref is swept to 5 V. The measurement was performed over a -40° to +85°C ambient temperature range. The result clearly shows the bias current changes little over temperature. The current mirror bias circuit also provides power down capability.

As shown in the data plot, when Vref is less than 2 V, the RF transistor is turned off. The power down capability allows the user greater flexibility in system design.

The ECP family features a single-stage design approach that provides flexibility for its users. ESD protection is included on-chip, protecting the circuit against 2000 V HBM (human body model). The ESD protection is not available with other discrete component solutions.

Partial Input Matching Circuit

As shown in the block diagram, the RF HBT is matched at the input, on-chip. For the ECP200 example, this input matching circuit is designed for the 2.14 GHz, W-CDMA base station transmitter frequency band. For lower frequency bands, the on-chip input matching circuit offers partial matching. Further matching made on the PCB will bring the input return loss to the desired level.

Table 2
The Amplifier's Class A and AB Characteristics

Class A

Class AB

Highest OIP3

Lower OIP3 than Class A

Higher power dissipation at back-off of RF power

Lower power dissipation (higher efficiency) at back-off of RF power than Class A

NF and P1dB are similar

NF and P1dB are similar

Class A or Class AB Bias

Table 2 compares the class A and class AB operation of the amplifier. The major trade-off is the output third-order intercept point (OIP3) with the power dissipation (efficiency) in a back-off state. For applications where OIP3 is not the top priority, class AB offers higher efficiency.

Probably the most interesting feature of the ECP-series to the designer is the flexibility of bias selection that allows the user to operate the amplifier in either class A or class AB, or anywhere in between.

The bias current of an ECP amplifier can be easily set (selected) in two ways: (1) by setting the Vref voltage or (2) by adjusting the value of the external resistor on the Vref pin. Method 1 is clearly shown in the Icc/Vref data plot.

Icc versus Vref is a linear relationship and the quiescent current can be set at any desired level, with little variation over temperature.

The user can change the external resistor value (Method 2) to modify the slope of the Icc vs. Vref response if needed for varying control capability. Method 2 can be understood by examining the diagram shown in Figure 3 . Icq2 of the RF transistor Q2 is proportional to Icq1 according to the ratio of transistor size, A2/A1. When Icq1 (~Iref) is increased, Icq2 is increased accordingly. Since Iref~(Vref-2.5 V)/R, the quiescent current Icq2 can be adjusted by the resistor value of R, while keeping Vref at a fixed level.

By adding an external resistor on the PCB, the total value of R is increased. The reference current Iref and quiescent current Icq2 are reduced accordingly, changing the bias condition from class A to class AB.

The HBT breakdown voltage is over 20 V for BVcbo and over 10 V for BVceo. Therefore, the amplifier can operate at higher than 5 V collector voltage. However, the ESD protection circuitry limits the collector voltage to approximately 12 V, which means Vcc is limited to 7 V to avoid the kick-on of the ESD protection.

When the device voltage is raised, the quiescent current must be reduced accordingly so the total DC bias power is constant. For example, the ECP200 amplifier is biased at 5 V and 800 mA. At 7 V, the current must be reduced to 570 mA, so the total power dissipation is maintained at 4 W.

The input matching is only slightly affected by the change of the collector voltage. However, the load impedance should be adjusted according to

RL = (Vcc-Vk)/Icc

Since Vcc is increased and Icc is reduced, the load resistance will increase accordingly.

High Linearity

Many wireless communication systems require high linearity as a result of complicated signal modulation and multi-carrier applications.

As a result, the demand on OIP3 is ever increasing. A high OIP3 can be achieved by using either a higher output power transistor or a more linear transistor. The higher output power transistor requires more DC power consumption, which is undesirable. Therefore, a more linear transistor is preferred.

The ECP-series is a family of high linearity amplifiers. The linear figure of merit (OIP3-P1 dB) for EiC's InGaP HBT process has historically been from 15 to 17 dB.

This high figure of merit has been maintained for the higher power ECP-series devices and provides outstanding performance to satisfy many infrastructure applications.

Figure 4 shows the linearity results using a W-CDMA signal for a base station. Both class A and class AB (50 percent class A quiescent current) were tested. The adjacent channel leakage ratio (ACLR) for both bias conditions is similar at the same output power, but the efficiency is higher for the class AB mode.

Figure 5 summarizes the two-tone test results. The third-order intermodulation (IM3) in class A is better than class AB at lower power levels, until both reach 26 dBm single carrier level (SCL). Twenty-six dBm SCL corresponds to 32 dBm peak envelope power (PEP); therefore, above this power level, waveform clipping starts and the IM3 in both bias conditions becomes similar.

It can be seen from the reported test results that the ECP200 intermediate power amplifier, as well as its lower power bretheren, demonstrates outstanding performance. Now the RF designer has a truly cost-effective option for higher power amplifier requirements, a result of the company's leading edge InGaP/GaAs HBT technology. The devices can also be configured in parallel to meet even higher power out requirements, well under the cost of existing solutions.

EiC Corp., Fremont, CA (510) 979 8953, www.eiccorp.com. Circle No. 302

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